Binds to the X-box motif of MHC class II genes and represses their expression. May play an important role in regulating the duration of an inflammatory response by limiting the period in which MHC class II molecules are induced by interferon-gamma. Together with PABPC1 or PABPC4, acts as a coactivator for TERT expression. Mediates E2-dependent ubiquitination. Interacts with PABPC1 and PABPC4. Ubiquitously expressed, with highest levels in thymus. Ubiquitously expressed at 12 dpc and 14 dpc. The RING-type zinc finger domain interacts with an ubiquitin-conjugating enzyme (E2) and facilitates ubiquitination. Belongs to the NFX1 family.