FACS analysis and growth rates of primary and immortal CEF cells. A) Cell cycle analysis of pre-senescent passage 6 CEF, passage 95 SC-2, and immortal passage 280 DF-1 cells was performed using fluorescence-activated cell sorting (FACS) of propidium iodide-stained cells. B) To determine growth rates, primary CEF (passage 6) and immortal CEF (passage 43 SC-2, passage 95 SC-2, and passage 280 DF-1) cells were plated at a density of 1 × 105 cells/10 cm dish using conditions as described in Materials and Methods. Both primary and immortal CEF cells were counted each day for up to 6 days to determine cell numbers. The immortal DF-1 cells are only shown through day 4 due to their relatively higher cell numbers.