SPTLC1 and motor neuron disorder: Although all affected family members were carriers of the L39del variant, the index patient (I-1) showed an HSAN1 plus motor neuropathy phenotype, with elevated plasma 1-deoxySLs and a higher alanine to serine ratio due to low plasma serine levels (Figure 6, B–D, and Supplemental Figure 7A) compared with the other family members with pure motor neuron disease without sensory neuropathy.