Hox-7.1 is expressed after formation of the optic cup, marking the domain that will give rise to the ciliary body. These genes are differentially expressed during early mesodermal patterning in mouse embryo. These epithelial-mesenchymal interaction are required for msx genes expression. The binding efficiency was dependent on the sequence, relative orientation and spacing of the repeated motifs of response elements. RARs have a preference for 5 bp spaced motifs, whereas TRs and RXRs preferentially recognize motifs separated by 4 bp and 1 bp respectively.
Direct repeats are selective response elements for thyroid hormone, retinoic acid and vit.
Upon binding ligand, intracellular receptors for retinoic acid, thyroids and steroids act as dimeric transcription factors. Thyroid hormone response elements is identified that consist of a direct repeat, not a palindrome, of a half-sites. Unlike palindromic TREs, direct repeat TREs do not confers a retinoic acid response. Sp1 protein activates transcription from many eucaryotic promoters. The Sp1 protein activates a specific subset of promoters transcribed by RNA polymerase II in vertebrates.