ESR1 and breast cancer: These results are supported by, and are consistent with, several studies that suggest that the observed heterogeneity in ER status among HER2 related tumors is consistent with differences in the basal and luminal origins of HER2-driven breast cancers [3,11,63] as well as additional studies which report that basal-like tumors have higher levels of the hypoxia gene expression program compared to luminal breast tumors [55,64].