ERBB2 and cancer: One study found no difference in mean number of brain metastases between triple-negative (5.33), HER2-positive (4.71), or tumors that expressed either ER or PR (5.35) (P = .88).21 However, this study was limited by smaller sample size (n = 100) and did find that triple-negative and HER2-positive cancers had a shorter mean time interval to onset of brain metastasis than other subtypes (triple-negative: 25.3 months, HER2-positive: 19 months, ER-positive or PR-positive: 42 months; P < .01).21