SMAD2 and cancer: Results confirm that senescent HMEC have enhanced activity of the TGF-β signaling pathway as evidenced by upregulation of Smad2 and Smad4, p16INK4A [26], COL4A2 [27], MMP-2 [30], uPAR [31,32], CD44 (a cancer stem cell, EMT and migration/invasion-associated gene induced by both TGF-β and ionizing radiation in A549 and MDA-MB-231 cells [24,25]), and MGAT5, an enzyme required for the extracellular domain of TβRII to mediate receptor partitioning into membrane rafts and efficient entrance into caveolin-positive endosomes [33].