Subtraction-hybridization cDNA libraries (1 –4 ) are libraries enriched for sequences representing mRNAs whose expression in one biological source (e.g., tissues, cell lines) is different from a second source. Single-stranded cDNAs from both sources are allowed to hybridize so that sequences common to the two sources will anneal. The annealed, double-stranded DNAs are “subtracted” from the hybridization solution, leaving a subset of cDNA molecules enriched for sequences preferentially expressed (or repressed) in the biological source of interest. Figure 1 diagrammatically represents the scheme for subtraction hybridization currently employed in our laboratory.