ERBB2 and vitamin D deficiency: Because HER2-targeted therapies substantially improve prognosis in HER2-positive breast cancer and introduces a new mechanism of immunotherapy, we hypothesized that successful correction of vitamin D deficiency would be associated with improved disease-free survival (DFS) in patients treated with curative intent.<h4>Methods</h4>We performed a retrospective interventional cohort study of patients with early-stage HER2-positive breast cancer treated at Cancer Treatment Centers of America Midwestern Regional Medical Center from 2008 to 2014.