Spontaneous development of mammary gland adenocarcinomas in STAT1 deficient (STAT1-/-), STAT1 null (S1N), and STAT1-/- × RAG2-/- female mice. (A) STAT1-/- (n = 23, inverted red triangles), S1N (n = 26, inverted purple triangles), and STAT1-/- × RAG2-/- (n = 31, blue squares) mice succumbed to mammary tumors, whereas none of the age-matched wild-type (WT) 129S6/SvEv (n = 41, black triangles) or RAG2-/- (n = 33, yellow circles) mice developed the disease. P values were obtained with log-rank test by comparing STAT1-/- × RAG2-/- with WT mice (***P = 0.0003) and STAT1-/- or S1N with WT mice (****P < 0.0001). Curves between STAT1-/- and S1N mice are not different to a statistically significant degree (P = 0.24). (B) Multiparous STAT1-/- mice (n = 11, red) developed mammary tumors at a higher frequency and with shorter latency than nulliparous STAT1-/- mice (n = 12, black) (P < 0.0001).