B

PAX5 is a transcription factor whose expression is essentially restricted to the B-cell lineage and is actively repressed during T-cell development; it is absent from thymic T cells at all stages (Souabni 2002 PMID 12479824; Fuxa 2007 PMID 17600970). In contrast, CD1a is displayed by cortical thymocytes, CD34 marks very early thymic progenitors, and TdT is present in immature T cells undergoing V(D)J recombination (de la Hera 1989 PMID 2484962; Galy 1993 PMID 7688021).