Notch1-Hes1 signaling is required for increased clonogenic growth and drug resistance of p53-KO MM TICs. (A) Colony formation of ID1-KD, ID2-KD, and nontargeting scramble (Ctrl)-treated p53-wt (Scr) and p53-KO MM.1S cells. (B) Western blot analysis of NOTCH1 and HES1 expression in p53-wt (Scr) and p53-KO MM.1S cells after treatment with a GSI. (C) Colony formation of HES1-KD and nontargeting scramble (Ctrl)-treated p53-wt (Scr) and p53-KO MM.1S cells (n = 4). (D) Colony formation of GSI-treated p53-wt (Scr) and p53-KO MM.1S, MM.1R, and MOLP8 cells (n = 4). (E) Western blot analysis of HES1 expression after doxycycline-induced Notch1 intracellular domain overexpression in p53-KO MM.1S cells. (F) Colony formation of Notch1 intracellular domain overexpressing p53-KO MM.1S cells after GSI treatment (n = 6). (G) Colony formation of GSI-treated clinical MM non-del17p samples (n = 6) and del17p samples (n = 3). (H) Colony formation of p53-KO MM.1S cells after treatment with GSI (2.5 nM) and melphalan (Mel, 1 uM) or GSI and bortezomib (BTZ, 1.5 nM) (n = 5). (I) Melphalan IC₅₀ values of GSI-treated p53-wt (Scr) and p53-KO MM.1S cells (n = 3).