The polymerase chain reaction (PCR) is now commonly used in laboratories involved in research studies and clinical diagnostic work (1 ,2 ). A major advantage of PCR combined with reverse transcription (RT-PCR) is that it can be used to amplify and detect rare mRNA within a specimen. However, conventional RT-PCR cannot be used either to quantitate the frequency of cells expressing a particular mRNA or determine the cellular origin of the amplified signal. Both of these factors may be relevant in the interpretation of gene expression.
