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Cell survival means that a biological cell remains alive over a specified interval and under specified conditions. It differs from cell proliferation: a neuron can survive for years without dividing, while an increasing cell population can conceal simultaneous cell division and cell death. A cell viability assay measures a chosen property, such as retention of an intact cell membrane or continuing metabolism, rather than every possible future function. ^[ref]

The balance is actively regulated. Survival signals can restrain apoptosis; loss of a required signal can permit a death programme. In a cell-free experiment, removing cytochrome c suppressed apoptotic activity and adding it back restored that activity in the presence of dATP. This linked a normally compartmentalized mitochondrial protein to activation of an apoptosis pathway, rather than identifying survival merely with a high metabolic readout. ^[ref]

A biological growth factor is a signaling ligand that modulates cell growth, cell proliferation, cell differentiation or cell survival. Many are proteins recognized by cell-surface receptors. The resulting response depends on which receptors and intracellular pathways the target cell contains.

For example, epidermal growth factor engages the epidermal growth factor receptor, while FGF8 acts through FGF receptors. These interactions can alter gene expression through signal transduction. The ligand and its receptor are separate molecular components, so their distribution jointly determines where a signal can act.^[ref]