129S

129S refers to a group of closely related inbred laboratory mouse strains known collectively as the 129 strain family. These substrains were developed for genetic research and have become a standard background in mammalian genetics. Common substrains include 129S1/SvJ, 129S6/SvEvTac, and 129S7/SvEvBrd, among others. While they share a broad genetic lineage, each substrain exhibits its own distinctive characteristics, and genetic differences among substrains can influence experimental outcomes.

A defining feature of the 129 family is their historical suitability for embryonic stem cell derivation and germline transmission. 129-derived embryonic stem cells were among the first to contribute efficiently to the germline in chimeric mice, which significantly advanced gene targeting and the creation of knockout models. As a result, many targeted mutations were generated in a 129 background and later backcrossed to other strains to study gene function on a defined genetic milieu.

Because substrains differ genetically, researchers emphasize substrain- and backcrossing history when interpreting phenotypic results. The background can affect a range of traits and experimental readouts, so accurate reporting of substrain designation and breeding history is important for reproducibility and comparison across studies.

Today 129S strains remain widely used in genetic and developmental biology research, particularly in projects involving targeted gene modification, stem cell biology, and chimera generation. They are often employed as donor strains for genetic engineering and as reference backgrounds in studies that seek to understand gene function within a controlled genetic framework.