Haploids and double haploids are important resources for studyingrecessive traits and have large impacts on crop breeding, butnatural haploids are rare in animals.Mammalian haploids arerestricted to germline cells and are occasionally found in tumourswith massive chromosome loss.Recent success in establishinghaploid embryonic stem (ES) cells in medaka fish and miceraised the possibility of using engineered mammalian haploid cellsin genetic studies.