ESR1 and breast cancer: The experimental in vitro data presented herein strengthen the notion of the antiproliferative potency of the COX-2 inhibitor, celecoxib on hormone-dependent and -independent breast cancer cells in vitro, confirm the growth inhibitory activity of the SERM Ly117018 in ER-positive breast cancer cells, support the hypothesis of membrane initiated SERM signalling effects, and above all exclude drug antagonism of Ly17018 and celecoxib in the investigated ER-positive and- negative human breast cancer cells in vitro.