PGR and breast cancer: Lonaprisan, on the other hand, also halted proliferation of T-47D breast cancer cells blocking the S phase entry induced by E2 (Afhuppe et al. 2010), inducing G1 cell cycle arrest, which required the upregulation of p21cip1 triggered by lonaprisan-bound PR to the p21cip1 promoter (Busia et al. 2011).