Fixable viability dyes and their uses

A method for distinguishing dead cells from live cells in a cell sample, the method comprising:

• contacting the cell sample with a labelling compound of formula:
• wherein LABEL is a fluorescent label, wherein the labelling compound has
• a molecular weight of 714.69 g/mol and an excitation/emission wavelength of 402 nm/425 nm,
• a molecular weight of 1186.27 g/mol and an excitation/emission wavelength of 638 nm/652 nm, or
• a molecular weight of 1647.66 g/mol and an excitation/emission wavelength of 852 nm/877 nm,
• under conditions whereby the labelling compound binds free thiol groups on cell surface proteins on dead cells and live cells and preferentially crosses cell membranes of dead cells and binds free thiol groups on intracellular proteins in the dead cells;
• and
• detecting the label in the cell sample, thereby distinguishing dead cells from live cells in the cell sample by detecting increased labelling of the dead cells, as compared to live cells, and as compared to a control cell sample stained with an NHS ester compound based dye.