A C-to-T transition occured spontaneously in intron 1. This results in a new splice donor site (G-GT from G-GC), which alters splicing and, together with an upstream cryptic splice donor CAG, inserts a new 65 bp exon between exons 1 and 2. This leads to a frame shift and premature termination. The absence of protein expression was confirmed by immunohistochemistry on the outer plexiform layer.