"Deletion mutants" refers to organisms or cells that have undergone a genetic change where a segment of their DNA has been deleted or removed, resulting in the loss of specific genetic information. Full definition
Gene deletion mutants of the same pathway or with similar fitness alterations in steady conditions can largely differ in their response to dynamic conditions. (msb.embopress.org)
Systematic Genetic Analysis with Ordered Arrays of Yeast Deletion Mutants Tong AH, Evangelista M, Parsons AB, Xu H, Bader GD, Page N, Robinson M, Raghibizadeh S, Hogue CW, Bussey H, Andrews B, Tyers M, Boone C Science 2001 Dec 14; 294 (5550): 2364 - 8 - PubMed Abstract - PDF (baderlab.org)
In vitro genetic selection was used to identify mutations that reactivate a virtually inactive sunY deletion mutant. (science.sciencemag.org)