DRD4 and colorectal carcinoma: Co‐IP experiments confirmed that DRD4 could interact with 4 type I receptors and all 5 type II receptors of the TGF‐β family (formation of heterotetramers with combinations of 2 type I and 2 type II receptor molecules is essential for TGF‐β signaling pathway activation), indicating multiple possibilities for combinatorial activation and comprehensive TGF‐β family activation in CRC.