MXD3 is a member of the MXD family of basic-helix-loop-helix transcription factors that form heterodimers with the cofactor MAX, and were originally proposed as functional antagonists of MYC proteins in the MYC/MAX/MAD transcriptional network. While some MXD proteins are MYC antagonists by acting as transcriptional repressors to promote cell differentiation, MXD3 is an atypical member that plays a role in proliferation rather than differentiation. MXD3 promotes granule neuron precursor (GNP) proliferation through a Sonic Hedgehog-mediated pathway, it is expressed in a mouse model of medulloblastoma, and its knock-down (KD) reduced proliferation of the human medulloblastoma cell line DAOY.