Hox genes in amphibians and reptiles

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Hox genes play a massive role in some amphibians and reptiles in their ability to regenerate lost limbs, especially HoxA and HoxD genes.^[1]

If the processes involved in forming new tissue can be reverse-engineered into humans, it may be possible to heal injuries of the spinal cord or brain, repair damaged organs and reduce scarring and fibrosis after surgery.^[2][3] Despite the large conservation of the Hox genes through evolution, mammals and humans specifically cannot regenerate any of their limbs. This raises a question as to why humans which also possess an analog to these genes cannot regrow and regenerate limbs. Beside the lack of specific growth factor, studies have shown that something as small as base pair differences between amphibian and human Hox analogs play a crucial role in human inability to reproduce limbs.^[4] Undifferentiated stem cells and the ability to have polarity in tissues is vital to this process.