Results: A large-scale resource development project at the Bloomington Drosophila Stock Center has improved the
choice of deletions beyond that provided by previous projects. FLP-mediated recombination between FRT-bearing
transposon insertions was used to generate deletions, because it is efficient and provides single-nucleotide
resolution in planning deletion screens. The 793 deletions generated pushed coverage of the euchromatic genome
to 98.4%. Gaps in coverage contain haplolethal and haplosterile genes, but the sizes of these gaps were minimized
by flanking these genes as closely as possible with deletions. In improving coverage, a complete inventory of
haplolethal and haplosterile genes was generated and extensive information on other haploinsufficient genes was
compiled. To aid mapping experiments, a subset of deletions was organized into a Deficiency Kit to provide
maximal coverage efficiently. To improve the resolution of deletion mapping, screens were planned to distribute
deletion breakpoints evenly across the genome. The median chromosomal interval between breakpoints now
contains only nine genes and 377 intervals contain only single genes.