MPYS is a recently identified plasma membrane tetraspanner that is associated with major histocompatibility complex class II (MHC-II) and mediates its transduction of apoptotic signals.  It has also been found to be associated with VISA, a mitochondrial protein that acts as an adaptor in virus-triggered signaling.  MPYS also interacts with IRF3 and recruits the kinase TBK1 to the VISA-associated complex, acting as a critical mediator of virus-triggered IRF3 activation and interferon (IFN) expression.  It is thought that the binding of nucleic acid to the innate immune protein RIG-I causes complex formation between RIG-I, VISA, and MPYS.  This complex then recruits TBK1 to phosphorylate IRF3 which then directly activates IFN transcription.  At least three isoforms of MPYS are known to exist.