Most animals are diploid, but haploid-only and male-haploid (such as honeybee and ant) species have been described(1). The diploid genomes of complex organisms limit genetic approaches in biomedical model species such as mice. To overcome this problem, experimental induction of haploidy has been used in fish(2,3). Haploid development in zebrafish has been applied for genetic screening(2). Recently, haploid pluripotent cell lines from medaka fish (Oryzias latipes) have also been established(3). In contrast, haploidy seems less compatible with development in mammals(4,5). Although haploid cells have been observed in egg cylinder stage parthenogenetic mouse embryos(6), most cells in surviving embryos become diploid. Here we describe haploid mouse embryonic stem cells and show their application in forward genetic screening.