MTAP and Huntington disease: ,10 As the methylthioadenosine phosphorylase (MTAP) gene is located in close genomic proximity (165 kb telomeric) to CDKN2A on 9p21 and deletions of this gene frequently co-occur with CDKN2A HD,11 loss of MTAP protein expression has also been proposed as a promising surrogate marker for CDKN2A HD detection in CNS and non-CNS tumors.12–15