pDsRed-Express-C1 is a mammalian expression vector that encodes DsRed-Express, a variant of Discosoma sp. red fluorescent protein (DsRed; 1). DsRed-Express contains nine amino acid substitutions ,which improve the solubility of the protein and reduce the time from transfection to detection of red fluorescence (2). In addition, these substitutions reduce the level of residual green emission (2). When DsRed-Express is expressed in mammalian cell cultures, redemitting cells can be detected by either fluorescence microscopy or flow cytometry 8–12 hours after transfection (DsRed-Express excitation and emission maxima = 557 nm and 579 nm, respectively).Although DsRed-Express most likely forms the same tetrameric structure as wild-type DsRed, DsRed-Express displays a reduced tendency to aggregate (2). The DsRed-Express coding sequence is human codon-optimized for high expression in mammalian cells (3).