Large segments of DNA can vary in copy number between individuals. Such copy number variations (CNVs) contribute greatly to genetic diversity and are also thought to be associated with susceptibility or resistance to some diseases, including cancer. Simple Copy Number Determination with Reference Query Pyrosequencing (RQPS), featured in the September issue of Cold Spring Harbor Protocols, provides an assay for determining the copy number of any allele in the genome. The method, from Raphael Kopan and colleagues at Washington University, takes advantage of the fact that pyrosequencing can accurately measure the ratio of DNA fragments in a mixture that differ by a single nucleotide. A reference allele with a known copy number and a query allele with an unknown copy number are engineered with single nucleotide variations, and the ratio seen between these probes and genomic DNA reflects the copy number. RQPS can be used to measure copy number of any transgene, differentiate homozygotes from heterozygotes, detect the CNV of endogenous genes, and screen embryonic stem cells targeted with bacterial artificial chromosome (BAC) vectors. RQPS is rapid, inexpensive, sensitive, and adaptable to high-throughput approaches. As one of our featured articles, the protocol is freely available to subscribers and non-subscribers alike.