Mepazine hydrochloride (Pecazine hydrochloride) is a potent and selective inhibitor of MALT1 protease, with IC50 values of 0.83 μM for GSTMALT1 full length and 0.42 μM for GSTMALT1 aa 325-760. It effectively suppresses the activation of the NF-kB pathway specifically in activated B cell subtype of diffuse large B cell lymphoma (ABC-DLBCL) cells and makes it a promising candidate for the treatment of ABC-DLBCL.