PGR and breast carcinoma: MPA treatment of C4HD and T47D human breast cancer cells resulted in rapid phosphorylation of c-Jun and c-Fos, which was abolished by pre-incubation with the antiprogestin RU486 or by knockdown of PR expression with PR small interfering RNAs (siRNAs) (Figure 1A, B and Additional file1: Figure 1A).