Its emergence reflected the coalescence of several factors: the completion of the human genome project and an evolving awareness of intrinsic heterogeneity amongst individuals; emerging evidence of heterogeneity of treatment effect for treatments that targeted a single disease; and evidence that biologic heterogeneity in specific cancers—for example, expression of estrogen receptors or Her2/Neu in women with breast cancer—could be used to guide the use of therapies that targeted these tumor markers [93]. The gene discussed is ERBB2; the disease is breast carcinoma.