NFASC and proximal spinal muscular atrophy: Smigiel et al. (2018) report the identification of a homozygous truncating NFASC mutation affecting the fibronectin type III domain, specific to the Nfasc155 isoform in a child presenting with a very severe neurodevelopmental disorder resembling spinal muscular atrophy, while Monfrini et al. (2019) report a homozygous missense mutation in a case with autosomal recessive ataxia and a demyelinating neuropathy. Importantly, a human Mendelian disease caused by NFASC variants has been reported only in these two single individuals to date.