To ascertain whether Ctip inactivation can elicit mammary tumors, we monitored conditional-null (n = 21; 7 Ctipco/−/Wapcre/+ and 14 Ctipco/co/Wapcre/+) females, each of which had been mated to induce pregnancy, Cre expression, and mammary-specific recombination of the Ctipco allele. Here, RBBP8 is linked to breast cancer.