HOXA1 (HOMEOBOX A1), also called HOMEOBOX 1F, is a protein that in humans is encoded by the HOXA1 gene. In PA-1 teratocarcinoma cells, HOXA1 was induced by retinoic acid earlier than other HOXA cluster genes.The HOXA1 gene is mapped on 7p15.2. In vertebrates, the genes encoding the class of transcription factors called homeobox genes are found in clusters named A, B, C, and D on four separate chromosomes. Expression of these proteins is spatially and temporally regulated during embryonic development. A common polymorphism in the HOXA1 gene is associated with a susceptibility to autism spectrum disorder. Three-dimensional reconstructions of Hoxa1 -/- rhombencephalon revealed that it bears only 5 rhombomeric structures instead of the normal 7. A single maternal administration of a low dose of the vitamin A metabolite retinoic acid was sufficient to compensate the requirement for Hoxa1 function in Hoxa1-deficient mice.