ESR1 and breast cancer: However, PAX2 phosphorylation on serine 393 residue, a marker of its activation [18], was higher in luminal/ERα+ cell lines (MCF-7 and ZR-75-1 cells [8]) compared to non-luminal/ERα- cell lines (MDA-MB-231 and HS578T [8]) (Figure 1a,b), showing preferential activation of PAX2 in luminal breast cancer cells in vitro, as observed in vivo [14,15].