ESR1 and breast cancer: These findings are in alignment with the few prior studies that have examined P-Rex1 expression in human breast cancers, including our prior report that demonstrated a correlation between P-Rex1 and ER in primary breast tumors and breast cancer cell lines [5] Sosa et al. evaluated P-Rex1 mRNA levels in a gene expression microarray data set and reported elevated expression levels in primary luminal breast cancers, but low levels in basal-like (triple-negative) breast cancers [11].