Breast cancer related allele has incomplete penetrance. BRCA1/2 are highly associated with breast cancer. Many people carrying the mutation in their BRCA1/2 ended up developing breast cancer; however, not all of them developed breast cancer. A number of people are positive for BRCA1/2 mutation and do not develop breast cancer. This is because BRCA1/2 have an incomplete penetrance. The penetrance level can be difficult to determine because it can be modified by many factors such as age, environmental factors, other genes (polygenic), and epigenetic regulation.Breast cancer related allele has incomplete penetrance. BRCA 1 and 2 are highly associated with breast cancer. Many people carrying the mutation in their BRCA 1 or 2 gene ended up developing breast cancer; however, not all of them developed breast cancer. A number of people are positive for BRCA 1 or 2 gene mutation and do not develop breast cancer. This is because BRCA 1 and 2 genes have an incomplete penetrance. The penetrance level can be difficult to determine because it can be modified by many factors such as age, environmental factors, other genes (polygenic), and epigenetic regulation.