Certain progestins have been found to stimulate the proliferation of MCF-7 breast cancer cells "in vitro", an action that is independent of the classical PRs and is instead mediated via the progesterone receptor membrane component-1 (PGRMC1). Norethisterone, desogestrel, levonorgestrel, and drospirenone strongly stimulate proliferation and medroxyprogesterone acetate, dienogest, and dydrogesterone weakly stimulate proliferation, whereas progesterone, nomegestrol acetate, and chlormadinone acetate act neutrally in the assay and do not stimulate proliferation. It is unclear whether these findings may explain the different risks of breast cancer observed with progesterone, dydrogesterone, and other progestins such as medroxyprogesterone acetate and norethisterone in clinical studies.