A single synthetic oligonucleotide was employed as hybridization probe to detect and enable isolation of the human insulin-like growth factor I (IGF-I) gene from a human genomic DNA library. The synthetic oligonucleotide probe coded for the B-chain of IGF-I and was designed for expression in Escherichia coli. Despite numerous interspersed mismatches, the synthetic probe hybridized specifically with seven recombinant lambda phage containing almost the entire B-chain region of the human IGF-I gene. The usefulness of this approach was further demonstrated by the detection of lambda phage containing human preproinsulin, using A and B chain synthetic oligonucleotides, 90 and 63 nucleotides in length, as hybridization probes. The nucleotide sequence of the human IGF-I exon suggests that IGF-I is synthesized as a larger precursor molecule.