Cytological subdivision of the S phase of human cells in asynchronous culture.

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Abstract
A method is described for subdividing S phase cells in asynchronous cell cultures on the basis of replication band patterns produced in chromosomes by bromodeoxyuridine incorporation. The criteria used for cell classification are objective, requiring the presence or absence of specific bands on particular chromosomes, and therefore lead to subdivisions amenable to quantitative analysis and for comparative purposes. Two schemes are given: key 1, based on bands in chromosomes 2 and 5, leads to five sub-phases; and key 4, based on bands in chromosomes 3 and 4, leads to four sub-phases. The order of the sub-phases, though not their relative durations, is identical in the six primary cell cultures (four fibroblast and two lymphocyte) tested. The technique provides for a detailed study of the programme of chromosome replication in normal and abnormal cells which, in time, should produce new criteria for diagnostic purposes.