Similarly the iab regulatory elements of the bithorax complex that control the expression of abdominal-A and Abdominal-B, and consequently the identities of the 2nd–9th abdominal segments, are transcribed into ncRNAs in a spatially ordered pattern [129]. It has also recently been shown that 231 ncRNAs are expressed from the four human HOX loci in a spatially and temporally ordered progression along developmental axes, one of which (termed HOTAIR) from the HOXC locus controls expression of the HOXD locus in trans, as shown by siRNA-mediated knockdown experiments [13]. None of these ncRNAs has yet been specifically associated with a genetic variant in mammals, although in Drosophila there are many homeotic mutations that lie in regions encompassed by ncRNAs [12],[124], and Drosophila geneticists were clearly intrigued by them [130]. Moreover, as pointed out by Rinn et al. [13], the existence of such transacting regulatory ncRNAs may explain the observation that the deletion of the entire HOXC locus exhibits a milder phenotype than the deletion of individual HOXC genes [131].
