Figure 1: Effects of 1,25 vitamin D on muscle cells: molecular and nuclear pathways. 1,25 vitamin D binds the vitamin D receptor (VDR) in the nucleus, where it forms a complex with the retinoid receptor (RXR). The complex 1,25 vitamin D/VDR/XDR activates gene transcription, leading to known genomic effects. On the other hand, 1,25 vitamin D binds a putative membrane receptor which activates MAP kinase (MAPK) and phospholipase C (PLC) pathways leading to nongenomic effects.