The use of haploid cells in genetic screening. A primary interest in haploid cells is their use for generating mutations for assignment of gene function. In haploid cells, loss of function mutations can be readily generated as no complementation by the homologous chromosome set is encountered. Phenotypic exposure to various selection strategies can be used to investigate gene function in specific pathways. Alternatively, libraries of cells containing mutations in genes can be generated and characterized. Screening in cell culture is a distinct advantage in mammals where combination of mutations to homozygosity requires breeding efforts that are both costly and time consuming. ES, embryonic stem.