Binds differentially to the SH3 domains of certain proteins of signal transduction pathways. Binds to phosphatidylinositols; linking the hemopoietic tyrosine kinase fes to the cytoplasmic membrane in a phosphorylation dependent mechanism. Expressed in a variety of tissues including lung, liver, skeletal muscle, kidney and pancreas. Phosphorylated. Phosphorylation at Tyr-448 may stimulate the activity of the LYN kinase (By similarity). The disease is caused by variants affecting the gene represented in this entry. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay.