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The cell cycle phase distribution of two human lung cancer cell lines (HS 24 and HS 57) grown both to half-confluency and confluency was determined. Both cell lines were then synchronized by applying a thymidine block forcing them to stay in the S-phase. After removal of the thymidine block, which allows the cells to go then through their cell cycle phases, N-acylamino acylpeptide hydrolase (EC 3.4.19.1) activity was measured using N-acetylalanine-p-nitroanilide as substrate (N-acetylalanine aminopeptidase). At the same times that the enzymatic activity was measured, the cell cycle phase distribution was analyzed, in order to determine the cell cycle phase of N-acetylalanine aminopeptidase synthesis. However, the cell cycle phase of N-acetylalanine aminopeptidase synthesis could not be determined. This result was caused by the fact that the cells remained synchronous only for a short period of time.