A simple phenyl-substituted DIM derivative, 2,2’-diphenyl-3,3’-diindolylmethane 1 (DPDIM), inhibited the growth of MDA-MB-231 triple-negative breast cancer (TNBC) cells (IC50 ca. 10 μmol/L; triple-negative refers to breast tumor cells lacking ER/estrogen receptor and PR/progesterone receptor, and no HER-2 overexpression). Here, PGR is linked to triple-negative breast carcinoma.