Under the best fitting model, the probability that a woman diagnosed with breast cancer before age 40 years carries a BRCA1 mutation is exp(L)/[1+exp(L)] where the log-odds L is given by the formula L=−5.1544+2.0539 x+2.2750 y+0.9784 z and: x is 1 if the woman's tumour has high mitotic index and 0 otherwise; y is 1 if the woman's tumour has a trabecular growth pattern and 0 otherwise; and z is the number of the woman's first-degree relatives who have been diagnosed with breast cancer before the age of 60 years. The gene discussed is BRCA1; the disease is breast cancer.