We have identified and mapped 61 novel and previously described chromosome 17 and X genes, using a human placental cDNA library. These genes were isolated using a gene identification and mapping strategy based on reciprocal probing of arrayed chromosome specific cosmid and cDNA libraries. This strategy scans gridded cosmids for nuclear genes and isolates the expressed sequence by a cosmid to cDNA filter hybridization. Inherent to this approach is the advantage of identifying the corresponding genomic cosmid clone of a particular cDNA. The genomic and cDNA reagents can be used for fluorescence in situ hybridization (FISH) and polymerase chain reaction (PCR) based mapping to resolve map positions of cDNAs belonging to gene families and those associated with multiple chromosomes. The downstream utility of reagents generated by the reciprocal probing methods is demonstrated in our studies.