ABSTRACT The common marmoset provides a very relevant primate model for understanding the organization of the human nervous system and the diseases that affect it. Like humans, marmosets also demonstrate cooperative social behavior and have advanced cognitive processes, making them of great interest in the field for modeling developmental and psychiatric diseases and their therapies. They are also ideal for multigenerational genetic experiments as they give birth twice a year and mature faster than most primates. However, while the CRISPR/Cas9 system has been used to knockout genes and create knock-ins of single amino acids in a heritable manner in marmosets, it has been a challenge in the field to create germline transmissible models of gene reporters and trinucleotide repeat genes analogous to their murine counterparts. The very low efficiency of homologous recombination (HR) in primates has precluded knocking-in coding sequences by simply injecting Cas9 protein and a guide RNA into embryos during in vitro fertilization (IVF) as is done for creating knockouts. This limitation has prevented modelling of more genetically complex neurological diseases such as Huntington’s disease (HD) and for creating conditional reporters in marmosets, both of which are mainstays in the mouse neurogenetics field. In addition to low HR frequency, other barriers to creating germline transmission of knock- ins include the absence of a well annotated marmoset genome until recently, lack of protocols for derivation of ground state marmoset pluripotent stem cells (cjPSCs), the low percentage of marmoset pregnancies after embryo reimplantation, and a general deficiency of developmental biology expertise in the marmoset field. We propose to harness our labs’ expertise in developmental biology, IVF technologies, and transgenic stem cell biology to overcome this barrier to widespread use of marmosets. We aim to create transgenic knock-in cjPSCs, convert them into ground-state pluripotent stem cells and then inject them into IVF morula to create a chimeric founder marmoset that carries the modified genome. We then aim to screen the transgenic gametes from the founder marmosets to create the F1 progeny and use them to correlate the molecular-behavioral phenotype of HD. As proof-of-principle, we will focus on three knock-in reporter lines to broadly target excitatory, inhibitory, and peripheral neuronal populations. Together, if successful, our aims will result in creation of the first primate model with neuron-specific reporters, establish the marmoset as a valid model of HD, enable access to single- cell transcriptomic changes at the early stages of HD in a primate disease model, and finally correlate these molecular changes with the behavioral phenotype. These aims will provide fundamental insights into the biology of HD and the role of huntingtin protein in different classes of neurons. The outcome of this project will also influence a better understanding of poly-glutami...