HOMEOBOX GENE

Genes for Regulation of Vertebrate Morphogenesis and r required for positional identity of various structures along the body axis from the branchial area to the coccyx.
Genes at the 3-prime end control the cranial region and are expressed before those at the 5-prime end, which control the caudal region.
The potent teratogen, retinoic acid, can activate these genes prematurely leading to abnormalities in the hindbrain and limb buds.

Valproic acid is believed to preferentially alter the expression of homeobox genes called Hox genes. Disregulation of Hox-gene expression by valproic acid may prevent normal closure of the posterior neuropore . Interestingly, the affected Hox genes, Hox d8, d10, and d11, all control posterior structures. This corresponds with clinical observations that most neural-tube defects caused by valproic acid are in the lumbosacral area.
A homeobox is about 180 base pairs long.
Inshort,Hox genes determine where limbs and other body segments will grow in a developing fetus or larva. Mutations in any one of these genes can lead to the growth of extra, typically non-functional body parts in invertebrates, but usually results in spontaneous abortion in vertebrates. The Hox genes were first found in the fruit fly Drosophila melanogaster .