We have previously reported a transcript of the novel gene for human immunoglobulin superfamily containing leucine-rich repeat (ISLR). By additional screening of a human retina cDNA library, we isolated another type of transcript with a 5' UTR different from that of the previously reported type. Genomic sequencing of the ISLR gene revealed that these two types of transcripts, ISLR-1 and ISLR-2, originated from the same gene but are composed of different first exons. Because the entire open reading frame is contained in the second exon, these two transcripts produce the same protein. Radiation hybrid mapping linked the ISLR gene to AFM248yh1, which is in the critical region of Bardet-Biedl syndrome type 4 (BBS4) on chromosome 15. Sequence analysis of the ISLR gene in five BBS4 patients, however, showed no mutations, although a few polymorphic changes were detected. Cloning of the mouse homolog of ISLR (Islr) revealed that the predicted protein consists of 428 amino acids, 86% of which are identical to those of ISLR. The Islr gene was expressed in various mouse tissues, including retina, in which Islr mRNA was detected in the ganglion cell layer, the inner nuclear layer, and the inner segment of the photoreceptor.
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