As a part of the functional analysis of the region from the position of the fragile X mutation to the telomere of the long arm of the human X Chromosome (Chr), we have developed a number of different approaches to identify genes located in this area. We describe here a procedure allowing the rapid identification of expressed sequences based on the hybridization of radioactively labeled complex cDNA probes derived from different pig and human tissues to cosmid clones gridded onto nylon filters and to restriction fragments of these clones. This technique has allowed the identification of a number of differentially expressed sequences in cosmid clones covering most of the Xq27.3 to Xqter region. Using these sequences as hybridization probes, cDNA clones for new genes expressed in a tissue-specific manner were isolated. Applied to genomic regions defined by overlapping cosmid clones, this method will serve as a major component in our strategy to establish integrated physical and transcription maps.