Serine is the major substrate for mammalian SPT, reflected in the low levels of non-canonical SLs found in mammalian cells; increased usage of alanine and glycine by the SPT complex is caused by mutations associated with a sensory neuropathy HSAN1 (Gable et al., 2010; Harrison et al., 2018). This evidence concerns the gene AGXT and sensory peripheral neuropathy.