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. This gene is part of the A cluster on
chromosome 7 and encodes a DNA-binding transcription factor which may regulate
gene expression,
morphogenesis, and differentiation. Three transcript variants encoding two different isoforms have been found for this gene. Expression of HoxA3 in these cells affects the proper formation of the
thymus,
thyroid, and
parathyroid organs. While the gene does not seem to affect the proliferation or migration of the pharyngeal neural crest cells, it does appear to trigger
cellular differentiation events required to form these organs. Knockout of HoxA3 leads to failure in forming the thymus (
athymia) and parathyroid gland (aparthyroidism). Mutant HoxA3 also causes a reduction in thyroid size. While the
follicular and
parafollicular cells still differentiate, their numbers are reduced and they are not evenly distributed throughout the gland. Mutant HoxA3 models show similar
phenotypes as those seen in
DiGeorge's syndrome, and it is possible that the two are linked. == Regulation ==