Abstract The BTB Kelch family member gene LZTR1 is an upstream regulator of RAS/MAPK signaling and is mutated in both cancers and RASopathy syndromes. Lztr1 is an unusual RASopathy gene that is observed in both autosomal dominant and recessive examples of Noonan Syndrome. Mouse models to study the cellular impact of Lztr1 deficiency have been challenging due to the lethality of germline mutants and cardiac defects. Therefore, the role of Lztr1 in the central nervous system remains largely unknown. Our preliminary data from a newly generated telencephalon specific Lztr1 conditional mutant revealed enriched activation of the MAPK pathway in the white matter region containing the myelinating oligodendrocytes (OLs). In addition, Lztr1 conditional mutants show defects in the expression of stage specific markers of OL development. Single cell RNA sequencing reveal that Lztr1 is expressed in both progenitor and mature stages of OL development. Here we propose to develop new mouse models to address the role of Lztr1 in OL biology. In the first aim, we will test the requirement of Lztr1 during OL development by generating cell type specific conditional mutants at early and late stages of OL development. We will also generate a mouse model for the dominant Lztr1 mutation Y193H and develop an allelic series to test how Lztr1 gene dosage impacts OLs development. In the second aim, we will test if mis- expression of Lztr1 impacts the OL lineage by lowering RAS/MAPK signaling. We will also test if expression of Lztr1 improves aberrant RAS/MAPK activation and OL phenotypes in Nf1 RASopathy mouse models. Finally, we will identify a core RASopathy gene expression signature in the OL lineage by comparing transcriptomes of Lztr1 and Nf1 mouse models. These studies will identify the specific roles of Lztr1 during distinct stages OL development and provide new mouse models key to understanding Lztr1 signaling mechanism and RASopathy biology.