CDK9 and diffuse large B-cell lymphoma: In vitro treatment of DLBCL cell lines (U-2932, VAL, OCI-LY3) with AZD4573 led to a time-dependent reduction in phosphorylation of RNAPIISer2 (CDK9 target site) but not at Ser5, consistent with previous reports asserting that AZD4573 selectively inhibits CDK9 (Fig. 1A) [7, 8].