Testing is typically considered when a family shows one or more of these patterns:

*Two or more cases of gastric cancer in close relatives, with at least one being diffuse-type, at any age
*One case of diffuse-type gastric cancer diagnosed before age 40, even without other affected relatives
*A personal or family history of diffuse gastric cancer together with a specific type of breast cancer called lobular breast cancer (CDH1 gene mutations also raise this risk in women)
*A family history of cleft lip or cleft palate alongside diffuse gastric cancer (seen in some families with CDH1 gene mutations)
A genetic counselor can assess the specific family pattern and advise whether testing for the CDH1 or CTNNA1 genes is appropriate, since not every family history needs testing.