Among the 551 participants for whom global genetic ancestry could be assessed, having a higher percentage of West African genetic ancestry was associated with shorter breast cancer DFS among 246 participants in the hormone receptor (HR)–positive/human epidermal growth factor receptor 2 (ERBB2 [formerly HER2])–negative subgroup (hazard ratio, 1.45 [95% CI, 1.04-2.04]). The gene discussed is ERBB2; the disease is breast cancer.