Cytotoxicity in cell culture is typically expressed as LC50 the concentration of a given agent which is lethal to 50% of the cells. This number is dependent on the incubation time with the agent. Previously, a two-term exponential model has been proposed to describe cell growth after a cytotoxic event: y(t) = k, * exp(-d(1)*t) + k2 * exp(d(2)*t). A dose-response relationship was observed between the parameter k2 in this formula and the concentration of the cytotoxic agent etoposide, in high-grade glioma cells was independent of incubation time.In order to test if the model can be applied more generally, DAOY medulloblastoma cells treated with MS275, a histone deacetylase inhibitor, were used.The observed data fit the model well.The concentration at which k2 is reduced by 50% is called KC50. It provides a far better description of the cytotoxicity of an agent in a specific cell line than the traditional LC50.