HOXB1 and autism spectrum disorder: In summary, our data indicate that: (a) HOXB1 gene variants, either rare or common, do not exert large or moderate effects on affection status in autism spectrum disorders, while minor contributions cannot be excluded due to sample size limitations; (b) modulatory effects on head growth rates are compatible with the developmental roles of this homeobox gene: and (c) HOXB1 could influence the clinical autistic phenotype in the area of stereotypic behaviours.