The recent development of haploid cell lines has facilitated forward genetic screenings in mammalian cells. These lines include nearhaploid human cell lines isolated from a patient with chronic myelogenous leukemia (KBM7 and HAP1), as well as haploid embryonic stem cells derived from several organisms. In all cases, haploidy was shown to be an unstable state, so that cultures of mammalian haploid cells rapidly become enriched in diploids. Here we show that the observed diploidization is due to a proliferative disadvantage of haploid cells compared with diploid cells. Accordingly, single-cell-sorted haploid mammalian cells maintain the haploid state for prolonged periods, owing to the absence of competing diploids. Although the duration of interphase is similar in haploid and diploid cells, haploid cells spend longer in mitosis, indicative of problems in chromosome segregation. In agreement with this, a substantial proportion of the haploids die at or shortly after the last mitosis through activation of a p53-dependent cytotoxic response. Finally, we show that p53 deletion stabilizes haploidy in human HAP1 cells and haploid mouse embryonic stem cells. We propose that, similar to aneuploidy or tetraploidy, haploidy triggers a p53-dependent response that limits the fitness of mammalian cells.haploidy | embryonic stem cells | p53 | HAP1 | chromosome segregation T he main advantage of yeast as a model organism for genetic studies is the availability of haploid cells, so that the mutation of a single allele can suffice to reveal a phenotype. This approach has been of enormous importance for biomedical research in recent decades, as exemplified by the number of Nobel Prizes awarded to discoveries that used yeast as a model system, including the discovery of autophagy, telomeres, and the cell cycle (1). In any case, there are questions intrinsic to mammalian biology, such as stemness or differentiation, that are difficult to address using yeast as a model, and that could be answered by the availability of mammalian haploid cell lines.