Tetramethylrhodamine iodoacetamides (TMRIA) are thiol-selective reactive dyes that are used to label proteins via the cysteine residues. 5-TMRIA, the pure 5-isomer of TMRIA, is increasingly preferred for some particular applications since the mixed isomers of TMRIA may give different results from batch to batch due to the varying ratios and different reactivities of the two isomers. For example, 5-TAMRA is reported to predominantly label SH-1 (Cys-707) of the myosin heavy chain in skinned muscle fibers.