Penzes et al. showed that the Kalirin-Rac1-PAK pathway is another signaling cascade downstream of EPHB2 that leads to dendritic spine maturation (Penzes et al. 2003). Kalirin (KALRN) is a Rac-GEF expressed postnatally in neurons and localised to the postsynaptic densities at excitatory synapses, where it participates in the formation and maintenance of dendritic spines. In young hippocampal pyramidal neurons, KALRN links trans-synaptic signaling through ephrinB (EFNB) and EPHB receptors to spine formation and maturation. Upon EFNB activation, KALRN binds EPHB2 and translocates to the postsynaptic membrane.