Conditionally tumorigenic BALB/3T3 mouse cells (which produce tumors in BALB/c mice only under special conditions) cannot sustain DNA synthesis and consequently stop proliferating in media containing low concentrations (0-0.02 mM) of physiologically available calcium. By contrast, cells that have been neoplastically transformed (tumorigenic in mice without special assistance) in vitro by different oncogens, can sustain DNA synthesis and proliferate in such calcium-deficient media. The possible importance for tumor growth of an ability to withstand calcium deprivation is examined. It is suggested that this property may prove to be a reliable indicator of neoplastic transformation.
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