B3GALT4 and Parkinson disease: It is now known that there is a deficiency of GM1 (and other brain gangliosides, particularly GD1a) in the PD SN [58,59] and that this is at least in part related to decreased gene expression in the PD brain for key biosynthetic enzymes involved in the synthesis of a- and b-series gangliosides GM1 and GD1b (B3galt4) and GD1a and GT1b (St3gal2).