PAX5, also known as Paired Box 5, is a transcription factor that plays a crucial role in B-cell development and differentiation. It is part of the PAX family of transcription factors, characterised by a conserved paired box domain involved in DNA binding and protein interactions. PAX5 is essential for determining B-cell lineage commitment during haematopoiesis and is expressed in progenitor cells destined to become B cells, supporting their development from haematopoietic stem cells.