HOXA1 and human HOXA1 syndromes: Athabascan brain stem dysgenesis syndrome (ABDS) and Bosley-Salih-Alorainy (BSAS) syndrome are allelic HOX gene disorders caused by recessive, pathogenic loss-of-function variants in the gene HOXA1. The HOX genes are homeodomain containing proteins that are critical for anterior–posterior differentiation in the developing embryo.