Deletion of intron 1 sequences between exons E1 and 2 (extending from 71 nucleotides downstream of exon E1 to 128 nucleotides into exon 2 removing its 3' splice site, SpeI-ApaLI fragment) as well as the 5' splice site of exon 2 (20 nucleotides of exon and 10 nucleotides of intron).