The Full form of ISCS is Interval-Specific Congenic Strains. ISCS is a general experimental design that allows mapping of a quantitative trait locus (QTL) into a 1-cM interval is presented. The design consists of a series of strains, termed “Interval-Specific Congenic Strains (ISCS)”. Each ISCS is recombinant at a specific 1-cM sub-interval out of an ordered set of sub-intervals, which together comprise a wider interval, to which a QTL was previously mapped. It is shown that a specific and previously detected QTL of moderate or even small effect can be accurately mapped into a 1-cM interval in a program involving a total of no more than 1000 individuals. Consequently, ISCS can serve as the ultimate genetic mapping procedure before the application of physical mapping tools for positional cloning of a QTL. In genetics, two organisms that differ in only one locus and a linked segment of chromosome are defined as Congenic. Similarly, organisms that are coisogenic differ in one locus only and not in the surrounding chromosome. Unlike congenic organisms, coisogenic organisms cannot be bred and only occur through spontaneous or targeted mutation at the locus. Congenic strains are generated in the laboratory by mating two inbred strains (usually rats or mice), and back-crossing the descendants 5–10 generations with one of the original strains, known as the recipient strain.