DiTO-1 is chemically equivalent to TOTO-1 (TOTO is the trademark of Invitrogen). DiTO-1 is a carbocyanine dimer with green fluorescence similar to FITC. It is cell-impermeant and easily distinguished from Cy5 and rhodamines as a nuclear counterstain and dead cell indicator. It is non-fluorescent in the absence of nucleic acids, and generates a very bright fluorescence signal upon binding to DNA. DiTO-1 gives strong and selective nuclear staining in cultured cells and in paraffin sections. Simultaneous labeling with cell-permeable Nuclear Red LCS1 dye and cell-impermeant TOTO-1 can be used to assess cell viability. DiTO-1 and Nuclear Red both have much greater extinction coefficients than that of DNA-bound propidium iodide.