MMS19 (MMS19 nucleotide excision repair homolog),also known as MET18,is a 1,030 amino acid nuclear protein containing seven HEAT repeats that belongs to the MET18/MMS19 family. Via its interactions with TFIIH p80 and TFIIH p89 helicases,MMS19 plays a role in nucleotide excision repair (NER) and RNA polymerase II (Pol II) transcription. MMS19 may also function as a transcriptional coactivator of estrogen receptor. While ubiquitously expressed,highest levels of MMS19 have been found in testis. At least five distinct MMS19 protein isoforms exist,which are produced by alternative splicing events.