BRCA1 and neoplasm: Of the 452 tumours reviewed, 154 had mitotic indices between 0 and 9/10 h.p.f. and of these 1 (1%) was a BRCA1 mutation carrier, 115 had mitotic indices between 10 and 19/10 h.p.f and of these 1 (1%) was a BRCA1 mutation carrier, 111 had indices between 20 and 49/10 h.p.f. and of these 2 (2%) were BRCA1 mutation carriers and 72 had mitotic indices of 50 or more/10 h.p.f. (range 0–292/10 h.p.f.)and of these 25 (35%) were BRCA1 mutation carriers.