ERBB2 and breast cancer: Strengths of this study include a relatively substantial sample size for this rare subtype of breast cancer; ascertainment of study participants from a pathology database rather than from a genetics database; population unselected for hormone receptor or ERBB2 status; pathologic confirmation of MpBC by breast-specific pathologists rather than identification of MpBC based on International Classification of Diseases codes, which can pose a challenge for classification of rare subtypes.