Paired box (PAX) proteins are a family of transcription factors that play important and diverse roles in animal development. Nine PAX proteins (PAX1-9) have been described in humans and other mammals. PAX proteins are classified into four structurally distinct subgroups (I-IV) based on the absence or presence of a carboxy-terminal homeodomain and a central octapeptide region. Subgroup II (PAX2, 5, and 8) contains the octapeptide and a truncated homeodomain. Pax2 is important for stem cell survival and lineage commitment during development. PAX5 is required to promote differentiation of common lymphoid progenitors (CLPs) into B cells. PAX8 is involved in the development of thyroid follicular cells and the expression of thyroid-specific genes.
