The sequence variant c.764-1G>C is located in the acceptor splice site of intron 5 and interferes with the splicing process. In fibroblasts, the variant causes deletion of the first 15 nucleotides of exon 6 (r.764_778del15), resulting in a truncated protein (p.Val256_Gly260del5). Studies in transfected cells show that this truncated protein is inactive, not processed and retained in the ER.