Complex sentence: Progesterone takes part in the growth regulation of kinds of tumors, in part by its interactions with its intracellular receptors (PR); the mPRs have been found in cancer cells and tissues too.
Split into simpler sentences: Progesterone takes part in the growth regulation of different kind of tumors, in part by its interactions with its intracellular receptors (PR). MPRs have been found in cancer cells and tissues too.