Evaluation of random cDNA clones as probes for human restriction fragment length polymorphisms.

We constructed two human cDNA libraries and selected clones hybridizing with more than five fragments of digested genomic DNA. We assume that these cDNAs detect sequences belonging to gene families. Compared with cDNAs derived from mRNAs of other tissues, the cDNAs of lymphocytes contained a higher proportion of these selected species of cDNA. We assume that these extra cDNAs are tissue-specific. In parallel tests, cDNAs belonging to gene families detected more restriction fragment length polymorphisms than did genomic probes, due to the larger number of restriction sites that can be checked using one probe. However, the chromosomal assignment of these polymorphisms often proved to be very difficult. In addition, we noticed that the mean length of EcoRI fragments hybridizing with our cDNAs is greater than the mean length of fragments hybridizing with randomly chosen genomic probes, possibly due to methylation connected with the inactivation of related active gene sequences.