Message amplification phenotyping (MAPPing) is a sensitive polymerase chain reaction (PCR)-based technique for the rapid determination of the mRNA phenotype of small numbers of cells. Isolated mRNA is reverse-transcribed (RT) into DNA, and then the DNA fragments corresponding to proteins or genes of interest are specifically amplified by PCR. The technique, also called RT-PCR, has wide application in biology and medicine, and comparison with bioassays demonstrates that MAPPing saves significant time and material. The technique should be applicable to analyse mRNAs of virtually any tissue or cell type.