We report that HDAC1,2 inhibition decreases H4K91 ubiquitination during DNA repair in response to doxorubicin (a chemotherapy agent), overcomes the BBAP-mediated DNA repair and sensitizes the otherwise chemoresistant or refractory EZH2GOF DLBCL cells to doxorubicin (a chemotherapy agent). Here, DTX3L is linked to diffuse large B-cell lymphoma.