SMAD6 and breast cancer: SMAD4, SMAD2, SMAD3, SMAD7, SMAD1, SMAD6, and SMAD5 showed genetic variation in 1.8%, 1.2%, 1.1%, 0.8%, 0.7%, 0.6%, and 0.5% of patients with breast cancer, respectively (Fig. 1C).