For Kieth and Andrea Rieger, the mission began eight years ago, when colorectal cancer claimed the life of their daughter, Stefanie, at age 22. She was the youngest and fourth known family member on Kieth Rieger’s side to develop the cancer.

The Melville couple has raised money for hereditary colorectal cancer research. They’ve given speeches and spread the word to relatives, friends, co-workers and complete strangers. No matter where they go, they underscore to people the importance of having a colonoscopy.

Their fervent hope is to make a difference, to shine a spotlight on the forms of the disease caused by miscues that are indelibly scripted in a family’s DNA.

Hereditary colon cancer can strike early in life — often before age 50 and sometimes in the early 20s, as was the case for Stefanie Rieger. A small percentage of patients can be affected in their teens. One hereditary type of the disease doesn’t produce polyps, while another causes the development of hundreds to thousands of them.

An American Cancer Society report, published in late February, rocked the medical community and raised alarms about a surge in colorectal cancer risk, primarily among millennials. The research, however, failed to mention the population of young people whose families have been stalked over multiple generations by inherited forms of the disease.

Kim Miller, an epidemiologist at the cancer Society’s headquarters in Atlanta and an author of the report, told Newsday the data used in the analysis did not include figures on hereditary colorectal cancers.

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“We do not have data at the population level at this point to determine how many colorectal cancers in this age group are related to hereditary factors,” she said in an email.

She and her colleagues suggested that all of the cancers they analyzed in the report, even among people in their 20s, were the result of lifestyle issues — poor diet, a lack of exercise and obesity.

Families such as the Riegers say some people may not even realize they carry a DNA flaw that can lead to colorectal cancer. And taking stock of one’s genetics and genealogy are not the first thoughts that come to mind when handed a cancer diagnosis, they said.

“For me, it was kind of like putting the pieces together. My father had colon cancer. But growing up, I never thought about it. Then, I got colon cancer when I was 39,” said Kieth Rieger, who was diagnosed in 1993.

“I got surgery and chemo and I just moved on and didn’t think about a genetic connection. I figured that was it,” he said. “Then, in 2004, I got colon cancer again — and that’s when I started to think it was genetic.”

Rieger, 63, a lawyer accustomed to in-depth research, dug deeper into his family’s cancer history and discovered that his grandmother also had colon cancer. His research revealed that the malignancy had occurred earlier with each generation: His grandmother was in her 60s when diagnosed; his dad was in his 50s.

DNA testing showed that Rieger carries a gene for Lynch syndrome, one of the most common hereditary cancer syndromes. As a carrier, risk escalates not only for malignancies of the colon and rectum, but cancers of the brain, small intestine, stomach, pancreas and upper urinary tract.

For women, the syndrome also increases the likelihood of uterine and ovarian cancers. Rieger’s grandmother had breast cancer as well.

“This genetic thing I have can morph into other problems,” said Rieger, who also has battled kidney cancer.

Dr. Jules Garbus, a cancer surgeon and specialist in colorectal cancer at Winthrop-University Hospital in Mineola, said Lynch syndrome is an important hereditary cause of the disease. He always prods medical students training under him to ask questions and seek detailed medical histories of patients, he said.

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“A lot of patients don’t have symptoms, which is why family history is so important,” Garbus said of young people. “Abdominal pain and bleeding are thought of as common symptoms, but a lot of people don’t have that. And at a young age, symptoms are sometimes so subtle patients may not give them a second thought.

“A young woman might associate [abdominal] discomfort with her menstrual cycle,” he noted. “So that’s where the diligence comes in from the clinicians’ standpoint.”

Kieth Rieger remembers experiencing abdominal pain and extraordinary fatigue, signals that alerted him in 1993 that something was not right. His daughter, however, apparently had no symptoms until the cancer broke through her colon wall — a medical emergency that occurred while she was visiting Israel, thousands of miles from home.

“She was on a three-week birthright trip,” Andrea Rieger said, recalling the summer of 2008 when the phone rang at their home and they learned the devastating news.

“The doctor said, ‘You have a very sick daughter. She has colon cancer and we are rushing her into surgery,’ ” Andrea Rieger said.

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Treatment in Israel and at Memorial Sloan Kettering Cancer Center in Manhattan could not save the young woman’s life. Stefanie Rieger died on Feb. 27, 2009.

Not everyone with Lynch syndrome develops colorectal cancer, but the lifetime risk is unusually high — about 80 percent, data from the National Cancer Institute show.

Medical scientists at Memorial Sloan Kettering Cancer Center estimate that 5 percent to 10 percent of all colorectal cancers are caused by a heritable mutation. In addition to Lynch syndrome, there are several other genetic causes of the cancer.

For high-risk families, it is generally recommended that routine colonoscopy screening begin a full decade earlier in life than the age of the closest relative who was diagnosed with the disease, said Dr. Robert C. Amajoyi, a surgeon and specialist in colorectal cancer at South Nassau Communities Hospital in Oceanside.

Under that scenario, Stefanie Rieger would have initiated screening at 29, a decade younger than her dad was when he was diagnosed. As it turned out, to start screening even at that age would have been too late for her.

Both Riegers said they encourage families at high risk to begin screening as early as possible, even in the teen years.

Nationwide, the incidence of colorectal cancer is increasing for people under 50, according to the American Cancer Society’s research, published in the Journal of the National Cancer Institute on Feb. 28.

The scientists projected that 135,430 colorectal cancers in people of all ages are expected this year, most of them in people older than 50. Of those, 13,500 colon cancers are expected to be diagnosed in people under 50, their report said.

Miller, a cancer epidemiologist, described cases in the study as sporadic colorectal malignancies, which means they occurred by chance and were not linked to a definable cause, such as a specific gene mutation. The conclusions drawn about lifestyle and obesity are a hypothesis, she said.

The study focused on the millennial generation, now in their late teens through early 30s, and members of Generation X, many of whom are under 45.

Generally, Lynch syndrome is responsible for 3 percent to 5 percent of all cases of colorectal cancer annually in the United States, according to the National Cancer Institute. It is more specifically known as hereditary nonpolyposis colorectal cancer, or HNPCC, because it doesn’t produce polyps.

By contrast, another genetic cause of the disease is well-known for creating polyps: familial adenomatous polyposis, or FAP, Garbus said.

He has performed colonectomies — colon removal surgery — on young Long Island patients with FAP mutations, some in their teens. Gene carriers can develop hundreds to thousands of polyps. Minimally invasive surgery, especially with robotic procedures, have greatly aided physiological and cosmetic outcomes for these patients, Garbus said.

He and his surgical team reroute a segment of the small intestine to function as a colon, eliminating the need for a colostomy bag.

Sharona Cohen, director of the cancer genetics program at Northwell Cancer Institute, said genetic counseling is critical for people who are at risk of a hereditary cancer. But neither genetic testing nor counseling are for everyone who develops cancer, she said.

“Ninety percent of cancers are not hereditary,” Cohen said. “Genetic counseling and testing should be recommended only when there are certain red flags,” such as multiple instances of certain cancers in families that occur before age 50.

Kieth and Andrea Rieger, meanwhile, fight back grief as they recall how their daughter’s cancer developed stealthily, without obvious symptoms.

Stefanie Rieger had graduated from Binghamton University shortly before her trip to Israel in the summer of 2008, with plans to attend law school that fall, Andrea Rieger said. Upon her return home, the young woman was scheduled to undergo a colonoscopy.

The screening was supposed to be a precaution the family thought would ensure a long and healthy future, Andrea Rieger said. No one knew that the cancer already existed and had spread, metastasizing beyond the colon.

“I don’t think there’s been enough being done by the medical community to alert younger people about this cancer and the need to be tested,” she said. “It’s a lot less expensive to be tested than it is to be treated.”