This ENU-induced A to T transversion of nucleotide 167 of the coding sequence results in a histidine to leucine change at amino acid 56, a highly conserved residue in vertebrates. This mutation increases the serine palmitoyltransferase (SPT) affinity toward the C18 fatty acyl-CoA substrate by twofold and elevates 20-carbon (C20) long chain base (LCB) production in the mutant mouse brain and eye.