ESR1 and breast cancer: On the other hand, the antiproliferative activity seems to depend on the structure of the PA chain (PATs Agel 416 and HO-416b also from spider venom), with PAT HO-416b being ca six times more active than PAT Agel 416 in the estrogen receptor alpha (ERα)-positive MCF-7 breast cancer cell line with low metastatic potential, but slightly less active in the ERβ-positive MDA-MB-231 highly invasive breast cancer cell line [6].