To model the effect of Pten loss on BC, we used a floxed allele (Ptenf) (Suzukiet al, 1998) and the deleter linesWAP-Cre (which preferentially targets pregnancy-identified alveolar progenitors) andMMTV-CreNLST (which targets basal and luminal progenitors) (Wagneret al, 2002; Jianget al, 2010).MMTV-Cre:Ptenf/f mice developed mammary tumors after a long latency of 26.4 monthswith incomplete penetrance (70%) (Fig1A).WAP-Cre:Ptenf/f females developed tumors with shorter latency (15.2 months) andalmost complete penetrance; by 18 months, nearly all mice had succumbed to cancer. The gene discussed is PTEN; the disease is breast cancer.