Another PKA inhibitor Metadoxine33 and the MSK1/2 inhibitor SB-747651A,34 which also inhibit the phosphorylation of CREB1/ATF1,35 suppress the sphere formation of breast (BT-549, MDA-MB-231) and lung (A549) cancer cells (Supplementary Fig. S2l–n). Here, CREB1 is linked to cancer.