LONDON-Lacy Anderson has a rare form of stomach cancer, but she is not alone.

In fact, all but one member of her family of nine share her same diagnosis.

One Anderson has already succumbed to the cancer. Six are now cancer free, but only because they had their stomachs removed altogether.

Lacy's is the final battle with hereditary diffuse stomach cancer. 

The disease is the result of a genetic mutation, affecting only abut 400 known families; 56 and 70 percent of members of a family with the mutation develop the cancer; in Lacy's family, it has struck nearly 90 percent. Despite her genetic profile, doctors dismissed 28-year-old Lacy's stomach troubles as 'stress,' she says, and it her cancer was already stage four by the time it was finally diagnosed.  

Lacy has idea what her prognosis is, so she is marrying her boyfriend of two years on Saturday, and drawing on the strength of a family that understands her illness like no one else could as she undergoes chemotherapy and fights for her life. It started with Uncle Frank.

Every time Lacy's father's brother got a colonoscopy, doctors found polyps all over his colon.

Suspecting colon cancer or another illness, his doctors ran blood tests, and found out that he had a mutation on his CDH1 gene, quite by accident.

Once his doctors knew he had the mutation, they knew Frank's cancer was almost certainly hereditary gastric diffuse cancer (HGDC). He was diagnosed in June of last year.

And once they knew that Frank had the mutation, his two children, Laura and Eric, got tested for it, and were positive.

The same story played out for Lacy's father, John, and her older brother, Robert.

'It just kept getting scarier and scarier,' Lacy told Daily Mail Online.

There are about 120 mutations that can happen along the CDH1 gene, and any of them can cause the rare stomach cancer because the gene normally codes for a tumor suppressing protein.

These mutations can also cause breast and ovarian cancers, but most commonly tumors begin in the stomach.

HGDC gets its name from the pattern of its tumor growth.

It is marked by a series of small clusters of cancer cells distributed sporadically throughout the stomach lining.

Endoscopies typically examine the upper part of the stomach, from where the cancerous clusters are not visible. Colonoscopies don't catch it either.

Coupled with the fact that it is rare, and therefore not at the top of the list of usual suspects most doctors look for, this means that HGDC often doesn't get diagnosed until it is in its late stages, when the survival rate may be as low as 20 percent.

Such was the case for Lacy's uncle, Frank, who died in May at age 63.