Serial analysis of gene expression (SAGE) studies often yield numerous tags that cannot be mapped to known gene sequences. Intriguingly, these may represent unknown genes, unknown parts of genes, or transcript variants. In order to elucidate the origin of these tags, 3 - and 5 -rapid amplification of complementary DNA ends (RACE) reactions can be performed using primers identical or complementary to SAGE tags. This way, transcript fragments, or indeed the entire uncharacterized transcript, can be cloned and sequenced.