CCND1 and neuroblastoma: Cyclin D1 is a marker of cell cycle entry and a key regulator of the G1-S transition, and its increased expression was associated with OPCs proliferation, both in vitro as well as in vivo. Interestingly, cyclin D1 is one of the early genes activated in OPCs treated with neuroblastoma-conditioned medium (37), and its expression is increased under conditions associated with high OPCs proliferation rates, such as in oligodendroglial tumors (38).