Acts as an inhibitor of BTK tyrosine kinase activity, thereby playing a role in B-cell development. Down-regulates BTK kinase activity, leading to interference with BTK-mediated calcium mobilization and NF-kappa-B-driven transcription. Interacts with the PH domain of BTK. Isoform 2 does not interact with BTK. Translocates to the plasma membrane upon IgM stimulation. Expressed in DeFew, HEK293T, HeLa and in Jurkat, MC3 and NB4 lymphoid cells (at protein level). Isoform 1 is the predominant isoform expressed in all examined tissues and cell lines. Highly expressed in hemopoietic tissues (fetal liver, spleen, lymph node, thymus, peripheral blood leukocytes and bone marrow). Weakly or not expressed in other tissues. Due to a partial intron retention. Due to a partial intron retention. Aberrant splicing.