Chapsyn-110 can cocluster with NR1 but also is concentrated at the axon initial segment. Neurons fixed at 5 (A, B) and 35 (C,D) d in culture were immunostained for NR1 (A, C) and chapsyn (B,D). Like PSD-95, chapsyn clustered with NR1 at some sites early in development (A, B,arrow) but also could cluster separately. In mature neurons, chapsyn coclustered with spiny NR1 (C,D, arrow) but also was concentrated at the axon initial segment without concentrations of NR1 (C, D, arrowhead). Scale bar, 20 μm.