ESR1 and breast neoplasm: To determine if genes within the c-Myb gene signature overlap with the luminal intrinsic gene set, we used the published human breast tumor microarray data (n = 232) of Herschkowitz et al., 2007 [29], clustered the tumors using the ∼2,000 intrinsic gene list described by Parker et al., 2009, and defined the luminal cluster as genes highly correlated (0.65 node correlation; 79 genes total) with ESR1, a central gene in the luminal cluster (Fig. S3 and Table S3).