To determine whether reduction of NO level contributes to breast cancer pathogenesis in vivo, we pharmacologically inhibited NO production in inbred wild-type mice (BALB/c) for a duration of 6 weeks (from 4 to 10 weeks old) by intraperitoneal (i.p.)injection of L-NAME (NOS antagonist, 20 mg/kg), in comparison to control (vehicle) and L-arginine (NOS substrate [agonist], 20 mg/kg) treatments. This evidence concerns the gene NOS1 and breast cancer.