Department of Pharmacology, University Medical Center at Stony Brook, Stony Brook, New York 11794-8651 Most attempts to identify and isolate a novel cDNA result in the acquisition of clones that represent only a part of the mRNA's complete sequence. The missing sequence (cDNA ends) can be cloned by PCR, using a technique variously called rapid amplification of cDNA ends (RACE), (1) anchored PCR,(2) or one-sided PCR. (3) Since the initial reports of this technique, many laboratories have developed significant improvements on the basic approach. (4-16) I will describe here the most recent hybrid version of the relatively simple classic RACE, and a more powerful but technically more challenging new RACE protocol that is adapted from the work of a number of laboratories. ~ 24) Commercial RACE kits that are convenient but not as powerful as the most recent versions of classic and new RACE are available from Bethesda Research Laboratories(~l) and Clontech.