Current means of measuring proteins in cells are relatively insensitive and do not provide a quantitative measure of the proteins present. To address this problem, researchers at the University of Pennsylvania (Philadelphia, PA) have developed a technique that is sensitive enough to detect the presence of a protein in a single cell (Proc. Natl. Acad. Sci. 8, 5497–5502, 2001). The technique—called immunodetection by T7 RNA polymerase or IDAT—combines the specificity of antibody-based detection of proteins with the amplification possible with PCR. The researchers linked Roche's (Basel, Switzerland) Herceptin antibody to a double-stranded oligonucleotide bearing a promoter region for the enzyme T7 RNA polymerase. The antibody–oligonucleotide complex binds to the target antigen, and then RNA polymerase generates multiple transcripts. The RNA polymerase amplifies the signal in a linear manner, so the concentration of the transcript produced is directly proportional to the amount of target antigen present. IDAT detected target antigen with a billion times greater sensitivity than existing techniques, and could even be used to distinguish between the different phosphorylation states of a specific protein. James Eberwine, lead author, says that the challenge will be to develop a bioinformatics platform that can marry data from IDAT, messenger RNA expression, and other biological assays.