POU2AF1, also known as BOB1, is a lymphocyte specific transcription coactivator protein. This protein interacts only with the Oct1/2 proteins through sub domains in the POU domain of the Oct1/2 proteins, enhancing their transcriptional efficacy. Although having no intrinsic capacity for DNA binding, POU2AF1 associates tightly with the octomer motif in the presence of Oct1 and Oct2. POU2AF1 is expressed at highest levels in spleen and peripheral blood leukocytes.