May have a special functional role during embryogenesis and in adult hematopoietic cells. Binds to ubiquitin. Preferentially expressed in adult hematopoietic tissues. High levels in lymphoid and myeloid tissues. During embryonic development it is expressed in both hematopoietic and nonhematopoietic tissues. By retinoic acid. Likely target of the activated retinoic acid receptor alpha. Belongs to the LAPTM4/LAPTM5 transporter family.