ERBB2 and breast cancer: The difference in trends between HER2− and HER2+ breast cancer was statistically significant for recreational physical activity only in the first 10 years after menarche (test for homogeneity of trends: P = 0.03) and was marginally statistically significant for physical activity in which the woman engaged at ages 10–19 years (test for homogeneity of trends: P = 0.06), whereas the trends in risk did not differ statistically for activity at ages 20–34 or 10 years before reference date (both tests for homogeneity of trends: P ≥ 0.12).