Signaling adapter that contributes to B-cell development by linking B-cell receptor (BCR) signaling to the phosphoinositide 3-kinase (PI3K)-Akt signaling pathway. Has a complementary role to the BCR coreceptor CD19, coupling BCR and PI3K activation by providing a docking site for the PI3K subunit PIK3R1. Alternatively, links Toll-like receptor (TLR) signaling to PI3K activation, a process preventing excessive inflammatory cytokine production. Also involved in the activation of PI3K in natural killer cells. May be involved in the survival of mature B-cells via activation of REL. Homooligomer (By similarity). Interacts (phosphorylated on tyrosine residues within YXXM motifs) with PIK3R1 (via SH2 domain); required for BCR- and TLR-mediated activation of phosphoinositide 3-kinase. The DBB domain is required for dimerization. Constitutively phosphorylated (By similarity). Phosphorylated on tyrosine residues within the YXXM motifs by BTK and SYK. Isoform 1 and isoform 2 are phosphorylated on tyrosine residues, most likely within the YXXM motifs, via CD19 activation. It is unsure whether this isoform is produced by alternative splicing or alternative initiation.