Effects of THZ1 on RNA Pol II phosphorylation and MCL1 expression. (A) U266 and MOLP8 cells were treated with various concentrations of THZ1 for 24 hours. Lysates were prepared and subjected to immunoblot analysis to assess S5 Pol II, S7 Pol II, Total Pol II, and CDK7 levels. β-actin was assayed to ensure equivalent loading and transfer. (B) U266 and MOLP8 cells were exposed to venetoclax (ABT199) ± THZ1 for 24 hours. Lysates were prepared for western blotting to assess the expression of apoptosis-related proteins, with β-actin used as a loading control. ABT, Abbott Laboratories; S5 Pol II, RNA Pol II at serine 5; S7 Pol II, RNA Pol II at serine 7.