During vegetative growth, Dictyostelium cells secrete the proteins AprA and CfaD, which inhibit the proliferation of Dictyostelium cells in a concentration-dependent manner. Extracellular levels of AprA and CfaD increase as a function of cell density, and cells lacking either AprA or CfaD proliferate more rapidly than wild-type cells, are multinucleate, and reach a higher MK-1775 stationary density than wild type. The addition of either recombinant AprA or rCfaD to wild-type cells slows proliferation, though cells lacking AprA are not slowed by rCfaD, and cells lacking CfaD are not slowed by rAprA, suggesting that these proteins require each other for activity. Cells lacking AprA or CfaD accumulate mass on a per nucleus basis at a rate like wildtype cells, indicating that AprA and CfaD regulate proliferation but not cell growth. As cells tend to starve when they reach high cell densities, slowed proliferation due to AprA and CfaD combined with unchanged cell growth may provide cells with stored resources that aid in survival under conditions of starvation. Basic leucine zipper transcription factors are a large family of proteins that function in a wide range of signal transduction pathways and are defined by an approximately 30 amino-acid sequence with a leucine residue at every seventh position, which mediates homo- or heterodimerization through alpha-helical interactions and a stretch of adjacent basic residues that mediate DNA binding. The Dictyostelium genome encodes 19 predicted bZIP transcription factors. Little is known regarding the function of the majority of these bZIP proteins, though the bZIP proteins DimA and DimB are downstream effectors of the prestalk cell fate-specifying signal DIF and translocate to the nucleus in response to this signal. The BzpN-independent branch may not be active in low-density cells, which would explain the inability of high-density conditioned media to arrest the proliferation of low-density bzpN2 cells. This model would predict the existence of mutants that proliferate like wild type cells at lower densities, but reach higher stationary densities than wild type. Mutants that show this phenotype have in fact been characterized. We observed that BzpN-GFP fusion proteins showed a punctate intracellular localization at low cell density, and addition of conditioned media from high-density cells resulted in a nuclear localization of BzpN-GFP, indicating that an extracellular signal present at high cell density induces a localization of BzpN at the nucleus. BzpN may play a role in mitigating cellular stress, as cells lacking BzpN show aberrant proliferation under conditions of nitrosative or thermal stress. Alternatively, BzpN may be functionally redundant with another protein affecting spore development or colony expansion, with the redundancy providing increased robustness.