Method for enumerating mammalian cell micronuclei with an emphasis on differentially staining micronuclei and the chromatin of dead and dying cells

A method for the enumeration of mammalian cell micronuclei, while distinguishing micronuclei from the chromatin of dead and dying cells, the method comprising:

• contacting a sample containing mammalian cells with a first fluorescent DNA dye that permeates dead and dying cells but not viable cells, that covalently binds chromatin, and that has a fluorescence emission spectrum;
• contacting the sample with one or more lysis solutions that result in digestion of mammalian cell outer membranes but retention of nuclear membranes, thereby forming free nuclei and/or micronuclei;
• contacting the free nuclei and/or micronuclei with RNase to substantially degrade RNA;
• staining cellular DNA with a second fluorescent DNA dye having a fluorescent emission spectrum that does not substantially overlap with the fluorescent emission spectrum of the first fluorescent DNA dye;
• exciting the first and second fluorescent DNA dyes with light of appropriate excitation wavelength;
• and
• detecting the fluorescent emission and light scatter produced by the nuclei and/or micronuclei, while excluding chromatin from the dead and dying cells, and counting the number of micronuclei in said sample relative to the number of nuclei.