Genome-wide collections of yeast knockout (YKO) mutants have facilitated the systematic functional analysis of yeast genes. An efficient method for analyzing knockout phenotypes in parallel is to pool knockout strains tagged with molecular barcodes and to measure their growth success in a test culture relative to a control culture by hybridization with barcode microarrays. However, haploid and homozygous diploid YKOs experience strong selection for compensatory mutations and so the genetic quality of such strains deteriorates over time. Knockout phenotypes can be protected from selection by “hiding” the mutation as a heterozygote with the wild-type allele. However, it is then correspondingly difficult to detect the knockout phenotype. High-quality knockout haploids can be created from heterozygotes by tetrad analysis, but this is labor intensive and unsuitable for genome-wide experiments. The Magic Marker technology (using the SGA reporter developed by Boone and coworkers¹) allows fresh creation of haploid knockouts from heterozygous diploids in a high-throughput manner by employing a simple selection step following sporulation². Thus, a starting pool containing thousands of high-quality MATa haploid YKOs can be efficiently created.