The ratio of RNA to total nucleic acid content as a quantitative measure of unbalanced cell growth.

Use of the metachromatic dye, acridine orange, to stain cells in suspension for flow cytometry allows for the simultaneous measurement of DNA and RNA content in individual cells. The relative RNA content as a function of total cellular nucleic acid content [alpha r = RNA/(RNA + DNA)] is a constant value, characteristic for particular cell lines during their exponential growth under optimal conditions. This ratio can be estimated for the G1A, G1B, S, and G2 + M cell cycle compartments. Changes in growth rate or the addition of antitumor drugs induces characteristic changes in the ratio either evenly throughout or at a particular phase of the cell cycle. Under such conditions, measurement of cellular DNA and RNA content provides a sensitive assay of any deviation from balanced cell growth. Unbalanced growth caused by suboptimal culture conditions or as a result of incubation with various antitumor agents is illustrated. Examples of unbalanced growth which are not correlated with cell viability as measured by cell clonogenicity are discussed.