The mediator complex is a multi-protein transcriptional co-activator that is expressed ubiquitously in eukaryotes from yeast to mammals and is required for induction of RNA polymerase II (pol II) transcription by DNA binding transcription factor.  One of the proteins in this complex is MED4.  This protein has also been shown to be part of the vitamin D receptor interacting complex.  MED4 was recently identified as an HIV dependency factor (HDF), suggesting that MED4 may be an important drug target in HIV treatment.  At least three isoforms of MED4 are known to exist; this antibody only recognizes the largest isoform.