Characterization of the CLCN5 5′UTR variant 4 and alternative variant 4 by RACE PCR and sequencing. + 1 indicates the putative transcriptional start site of alternative variant 4, located at intron 1a, 1001 nt upstream of the first nucleotide of exon 1b described by Fisher et al. (2). For variant 4 it was not been possible to obtain the full length cDNA but probably the two variants have the same transcription start site and differ only regarding the absence/presence of intron 1b. Coloured boxes represent exons and open boxes represent introns.