The Src-family kinases (SFKs) SRC and FYN, colocalised with ephrinBs (EFNBs) at the membrane, are required for EFNB phosphorylation. The cytoplasmic domains of all three EFNBs consist of five conserved tyrosine residues of which three undergo phosphorylation. Electrospray tandem mass spectrometry and site-directed mutagenesis identified tyrosines 312, 317, and 331 (human 324, 329 and 343) of EFNB1 as phosphorylation sites (Kalo et al. 2001, Palmer et al. 2002).