EphrinB proteins are thought to play key roles in cellular functions as diverse as neuronal migration and blood vessel development (Flanagan and Vanderhaeghen, 1998, Dufour et al., 2003, Oike et al., 2002). EphrinB molecules expressed at the membrane surface bind to the EphB family receptors on target cells during cell-to cell contact. This interaction leads to cell signaling in the target cell but also generates a reverse signal in the cell expressing EphrinB on its surface. This reverse signaling event is thought to be critical for vessel maturation and neuronal development. Importantly, tyrosine phosphorylation of EphrinB is thought to be a critical component of this reverse signaling event (Palmer et al., 2002). Recent work demonstrated that Tyr331 of EphrinB was phosphorylated in HEK293 cells after stimulation by the soluble EphB2 receptor tyrosine kinase (Kalo et al., 2001). Anti-Phospho-Tyr331 EphrinB Western blot of rat testes lysate showing specific immunolabeling of the ~46k EphrinB phosphorylated at Tyr331 (Control). The phosphospecificity of this labeling is shown in the second lane (lambda-phosphatase: (-Ptase). The blot is identical to the control except that it was incubated in (-Ptase (1200 units for 30 min) before being exposed to the Anti-Tyr331 EphrinB. The immunolabeling is completely eliminated by treatment with (-Ptase.