Malignant melanoma, the deadliest form of skin cancer, is characterized by a predominant mutation in the BRAF gene(1-3). Drugs that target tumours carrying this mutation have recently entered the clinic(4-7). Accordingly, patients are routinely screened for mutations in this gene to determine whether they can benefit from this type of treatment. The current gold standard for mutation screening uses real-time polymerase chain reaction and sequencing methods(8). Here we show that an assay based on microcantilever arrays can detect the mutation nanomechanically without amplification in total RNA samples isolated from melanoma cells. The assay is based on a BRAF-specific oligonucleotide probe. We detected mutant BRAF at a concentration of 500 pM in a 50-fold excess of the wild-type sequence. The method was able to distinguish melanoma cells carrying the mutation from wild-type cells using as little as 20 ng mu l(-1) of RNA material, without prior PCR amplification and use of labels.