May have an regulatory role in normal and neoplastic B cell development. The tyrosine-phosphorylated isoform 2 interacts with PTPN6. Expressed in the secondary lymphoid organs, spleen and lymph node. Expression is limited to the mature B-cell lines. Highly expressed in CD19 and within the mantle zones of the tonsil tissue. Isoform 2 is expressed in the spleen, peripheral blood and bone marrow. Isoform 2 and isoform 4 are expressed in B-cell lines. Preferentially expressed in memory B-cells (at protein level). Contains 2 copies of a cytoplasmic motif that is referred to as the immunoreceptor tyrosine-based inhibitor motif (ITIM). The phosphorylated ITIM motif bind the SH2 domain of PTPN6. Isoform 2 is N- and O-glycosylated, and phosphorylated.