The fluorescent dye silicon-rhodamine (SiR) which is structurally related to the rhodamine fluorophores (e.g. Texas Red), is a relatively small and bright fluorescent dye with a spectral range in the far-red fluorescent spectrum.

The red-shifted excitation wavelength can minimise phototoxicity and because of its excellent photostability SiR is particularly suited for high-resolution microscopy (e.g. STORM, STED) in living cells. Due to the long wavelength of red excitation, also a higher penetration depth and thus a more intensive imaging could be observed.

In contrast to other fluorescent dyes in this spectral range, the dye molecule SiR shows high membrane permeability in living cells, so that the use of fixed cells or the permeabilisation of cells, which is always associated with disruption of the cellular environment, can be dispensed with.
SiR is compatible with most microscopes as it can be used with standard Cyanine 5 filter settings.