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the percentage of cells that are replicating (making a copy of) their deoxyribonucleic acid (DNA). DNA replication usually means that a cell is getting ready to split into 2 new cells. A low s-phase fraction is a sign that a tumor is slow-growing; a high s-phase fraction shows that the cells are dividing rapidly and the tumor is growing quickly. See also deoxyribonucleic acid.