Pulse-chase is a technique used to monitor the rate of protein synthesis in vivo, by adding a 'pulse' of labelled 35S-methionine for a set time, and then 'chasing' the radio-label out by incubating cells with an excess of unlabelled methionine. Methionine is chosen because it is incorporated universally as the starting amino acid of all proteins. Cell extracts can be isolated at various time points in this process, proteins immunoprecipitated and then SDS-PAGE is used to quantify the 35S label.