The CYP21A2 gene travels in tandem with a pseudogene, CYP21P1, and the high degree of sequence similarity between them indicates that these two genes are evolving in tandem through intergenic exchange of DNA.[31] The CYP21A2 gene is located within the RCCX cluster, which is the most complex gene cluster in the human genome.[32] Due to the high degree of homology between the CYP21A2 gene and the CYP21P1 pseudogene and the complexity of the locus, it is difficult to study the CYP21A2 gene at the molecular level.[33]