HCN4 and breast cancer: Like Sema6a−/− retinas, in Plxna2−/−; Plxna4−/− double mutants, but not in Plxna2 or Plxna4 single mutants (data not shown), Syt2+ OFF type 2 CBC, HCN4+ OFF type 3a CBC and VGlut1+ BC axon terminations were disrupted (Fig. S8D,E), supporting plexin receptor redundancy in guiding BC axons.