A. HEK293T cells were transfected with Myc-tagged Ubiquitin (Myc-Ub), Flag-HDAC1, Flag-HDAC2, HA-USP19 WT, HA-USP19 CA as indicated. Cell lysates were extracted under denaturing conditions (95°C, 1% SDS) and immunoprecipitated with anti-Flag, immune complexes were immunoblotted with anti-Flag. B. Seventy-two hours after Control or USP19 siRNA transfection, protein levels of HDAC1, HDAC2 and USP19 were assessed by Immunoblot. C. HA-USP19 WT, HA-USP19 CA or HA vector was transfected into cells. Seventy-two hours after transfection, protein levels of HDAC1, HDAC2 and HA-USP19 were assessed by Immunoblot. D. 293T cells were transfected with Myc-Ub, Flag-HDAC1, Flag-HDAC2, HA-USP19 WT, and HA-USP19 CA as indicated. Cell lysates were extracted under denaturing conditions (95°C, 1% SDS), then immunoprecipitated with anti-Flag and immunoblotted with linkage-specific antibodies recognizing K63-linked polyubiquitin.