The present invention provides a variant proliferation-inducing ligand (APRIL), which has a higher binding affinity to BCMA than wild-type APRIL； and/or altered binding kinetics compared with wild-type APRIL, and/or a higher BCMA:TACI (transmembrane activator and calcium modulator and cyclophilin ligand interactor) binding ratio than wild-type APRIL and which comprises mutations at one or more of the following positions: A125, V174, T175, M200, P201, S202, H203, D205 and R206.
