Fig. 3. RPA at the pseudoautosomal region, PAR, of the X-Y chromosomes. (A) Chromosome cores and centromeres visualized with fluorescent tags. The X and Y chromosomes have a short paired region, PAR. (B) The same chromosome visualized with anti-RPA antibody. The autosomal SCs and the unpaired X-chromosome core have numerous RPA foci. The PA region frequently has the brightest RPA focus or foci. Scale: the SCs are about 15 mm in length. (C; insert) At early pachytene there is extensive non-homologous synapsis between the distal portions of the X and Y chromosomes. RPA is present in the most distal portion, presumably the actual PA region. (D; insert) In the rat, the late RNs are better defined than in the mouse, and the Figure illustrates a group of 5 nm RPA gold grains in association with a nodule.