PCR using primers of arbitrary sequence can generate a reproducible fingerprint of products from DNA (1 ,2 ). Differences in the fingerprint of products generated from DNA of related organism are a result of polymorphisms. These polymorphisms proved useful markers for genetic mapping (3 –7 ), population biology (8 –13 ), epidemiology (14 –19 ), and even the discovery of the mutator phenotype in cancer (20 ). When applied to RNA the method is also capable of detecting polymorphisms in expressed transcripts (21 ). However, more importantly, the method can detect a sample of differentially expressed genes (21 ,22 ).
