Historically, the 129 inbred mice are known for the high incidence of spontaneous testicular teratomas, though the incidence differs between substrains (1-3% in 129 parental substrains; 30% in teratoma substrains.) More recently, 129 mice are widely used in the production of targeted mutations due to the availability of multiple embryonic stem cell lines derived from them. There is major genetic variation within the 129 "family", which has led to an update of the nomenclature and a division of the substrains into three major groups: parental substrains (129P), steel substrains (129S) and "teratoma" substrains (129T). Investigators using 129 substrains for targeted mutagenesis should be careful in the selection of the appropriate 129 substrain to match the embryonic stem cell line. For a complete history of the numerous 129 substrains, see <a href="https://pubmed.ncbi.nlm.nih.gov/9140391/">Simpson, et al., 1997</a>. In 2019-2020, researchers at The Jackson Laboratory discovered this inbred strain contains the Trem2S148E allele - a naturally occurring variant at position 48351151-48351152 on Chr 17 (<a href="http://useast.ensembl.org/Mus_musculus/Variation/Sample?db=core;r=17:48351102-48351202;v=rs108080490;vdb=variation;vf=14473644">rs108080490</a> and <a href="http://useast.ensembl.org/Mus_musculus/Variation/Sample?db=core;r=17:48350652-48351652;v=rs107649577;vdb=variation;vf=14110567">rs107649577</a>; Ensembl GRCm38.p6). This TC to GA transition results in a serine to glutamic acid substitution at amino acid 148 (S148E).

129 mice are widely used in the production of targeted mutations due to the availability of multiple embryonic stem cell lines derived from them. They also have a high incidence of spontaneous testicular teratomas.
