A restriction-fragment–length polymorphism (RFLP) detected with the human insulin-receptor cDNA and the enzyme Sac I has been reported to be associated with non-insulin-dependent diabetes mellitus (NIDDM) in White and Black populations and segregated with diabetes in two small pedigrees with maturity-onset diabetes of the young. A size difference of ∼500 base pairs (bp) was demonstrated between the alleles of this and several other RFLPs that mapped to the same 300-bp region near the transmembrane coding region of the cDNA (β-chain, thus suggesting the presence of an insertion in this region that could affect insulin-receptor function. Genomic DNA fragments containing this RFLP were cloned from an individual heterozygous for the putative insertion, and the differing fragments of the two alleles were sequenced. The presence of a 400-bp insertion was thus confirmed and was demonstrated to be entirely within an intron. No significant coding-region differences from published cDNA sequences were detected in four exons sequenced from the region of the insertional allele. The sequenced regions included multiple Alu repeat sequences. The RFLP was unusual in that the larger allele consisted of an additional Alu repeat sequence that included a new Pst I site. Because the nature and location of the insertion did not suggest a role in insulin-receptor function, the association of this RFLP with NIDDM and hyperinsulinemia was reexamined in a small sample of Whites. No association could be demonstrated, and the insertion also failed to segregate with NIDDM in five White pedigrees. The lack of evidence for a role of this insertion by molecular, genetic, and physiological analyses suggests that this RFLP should be used cautiously as a marker for NIDDM, although the sequence data suggested that this region might be predisposed to recombination and mutation.

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