SOS1 and cancer: Using smooth muscle actin (SMA) immunostaining as a specific marker of cancer-associated fibroblasts (CAFs)44 in the TME of the lung tumors, we observed that SOS1 (but not SOS2) ablation dramatically reduced detection of SMA-positive CAFs in the TME of the lung tumors of 5-month-old KRASG12D-mice as compared to their SOS1/2WT and SOS2KO counterparts (Fig. 3a).