We have previously shown that skin fibroblasts from breast cancer patients display abnormal growth properties when compared with cells from patients with benign breast lesions. In the present study, we shown that it is possible to define, within the patients previously analysed, a subgroup whose fibroblasts exhibit a significant fraction of cells still synthesizing DNA when growth curves reach a plateau. The phenomenon can also be detected in skin fibroblasts from patients with other types of cancer. In two instances detection of this defect preceded the discovery of the disease. The high percentage of labelled interphases when cell counts reach a plateau is not due to an increased duration of S phase, relative to total cell cycle duration. The data suggest that these cells are delayed in the G2 phase. This assay buttresses our previous results, which suggested that at least in some instances cancer is a systemic disease; it could be used for the screening of patients at high risk of cancer. Research on the patients' somatic cells could shed more light on the process leading to neoplasia, rather than the study of the tumour cells that have already gone through several steps of the evolution to malignancy.

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