ENU mutagenesis induced in a T to A transversion at nucleotide 943 in exon 6 resulting in a cysteine to serine substitution at amino acid 315 in the Np65 isoform or amino acid 199 in the Np55 isoform. The mutated amino acid is located in the common extracellular Ig-like module 3 present in both the Np65 and Np55 isoforms. In vitro analysis indicates the mutant protein fails to localize to the plasma membrane and is instead retained in the endoplasmic reticulum.