A CTCT repeat located six base pairs 5' to exon 4 is replaced with a 225-bp repetitive sequence which is 100% homologous to a repeat element located 797 bp upstream. This interferes with the third intron 3' splice site resulting in exon3 spliced to exon 5. The resulting protein, lacking exon 4, is not secreted as is the wild-type allele. (J:105260)