CYP21 (also CYP21A, CYP21A1, CYP21B and CYP21a2, it’s seems they keep renaming it every so often) is the designation given to the gene which controls 21 hydroxylase (CYP is short for Cytochrome P450). The evidence of a defect in this gene is usually assumed when a deficiency in 21-hydroxylase enzyme is found.  (see http://www3.ncbi.nlm.nih.gov/htbin-post/Omim/dispmim?201910, http://www.imm.ki.se/CYPalleles/cyp21.htm, http://www.informatics.jax.org/searches/marker.cgi?1311, http://medicalgenetics.or.kr/journal/ina05.html, http://www.expasy.ch/cgi-bin/niceprot.pl?P08686, http://bioinfo.weizmann.ac.il/cards-bin/carddisp?CYP21A2&search=CYP21&suff=txt)

Are you wanting to know about the enzyme 21-hydroxylase or the gene CYP21?