Designed to detect the T790M mutation in exon 20 of the epidermal growth factor receptor (EGFR) gene, the assay will allow drug developers to investigate its reported association with acquired resistance and can act as a useful tool to predict drug response. By harnessing the sensitivity of ARMS allele-specific PCR and its Scorpions quantitative real-time signaling system, the assay can detect mutant copies even if they represent as little as 1% of the sample. This is a marked improvement in sensitivity over traditional sequencing-based techniques that lack discrimination when there are high levels of background DNA. The assay performs equally well using DNA extracted from various samples, such as paraffin embedded tumors and blood. Intended as a research tool, the mutation assay detects the presence of a mutant thymidine base in a background of normal cytosine bases at position 2369 of the EGFR gene. It is presented as two real-time PCR assays: one detects the mutant base the other acts as a control by quantifying the EGFR gene. Results are semi-quantitative and provide an approximate determination of the percentage of mutant T790M in the sample.