MAPK1 and neoplasm: For instance, the RAC1B-driven USP26-SMAD7-mediated degradation of ALK5 and the autocrine TGFβ1-SMAD3-BGN-mediated sequestration of exogenous, i.e., stromal cell-derived TGFβ is supposed to decrease TGFβ signaling, eventually resulting in downregulation of tumor-promoting MAPK/ERK and RAC1 signaling (Figure 4, left-hand side).