We have developed a family of mammalian coexpression vectors that permit identification of living or fixed cells overexpressing a gene of interest by surrogate detection of a coexpressed marker protein. Using these 'pMARK' vectors, a fluorescence-based, single cell proliferation assay was developed and used to study the effect of retinoic acid receptor beta (RAR-beta) on cell cycling. We demonstrate that transient overexpression of RAR-beta in the presence, but not absence, of all-trans retinoic acid results in a dramatic suppression of cell proliferation. We further show that this effect requires the DNA binding (C) domain of RAR-beta. It has been previously shown that RAR-beta expression is markedly altered in a variety of neoplasms and cell lines. Our data support the hypothesis that loss of RAR-beta may contribute to tumor progression by removing normal restraints on proliferation. The pMARK vectors should be useful for studying other genes that putatively suppress or enhance proliferation.