AP1S1 and MEDNIK syndrome: In this study, we demonstrated that the autosomal recessive MEDNIK syndrome, described in the population of Quebec, is caused by a founder mutation in AP1S1. More specifically, we have shown that the A to G mutation in the acceptor splice site of exon 3 of AP1S1 (IVS2-2A>G) was associated with skipping of this exon, leading to a premature stop codon.