Selective inactivation of Stat1 in mammary cells indicates that its effect as a tumor suppressor in breast is direct.

STAT1 is considered a tumor suppressor, but it is not known if this effect occurs directly in mammary cells or secondarily by disrupting interferon signaling through the JAK/STAT1 pathway to induce immune responses. ERBB2/neu-induced breast cancer appeared sooner in mice lacking STAT1 only in mammary cells than in wild-type mice, indicating that STAT1 tumor suppression was intrinsic to mammary cells and not secondary to an induced immune response.