Myotonic dystrophy type 1 (DM1), the most common adult-onset dystrophy, is caused by CTG repeat expansions (CTGexp) in the DMPK 3’UTR which are transcribed into toxic CUGexp RNAs that alter developmental RNA processing regulated by CELF and MBNL splicing factors. Unfortunately, key aspects of the variable and multi-systemic symptoms of this disease are still poorly understood. Additionally, current transgenic mouse models are not optimal for drug testing since they express interrupted repeats, fail to recapitulate the host gene spatiotemporal expression patterns, or are generally studied as homozygotes that fail to model the genetics of a dominant disease. To address this issue, we have previously generated Dmpk CTG480 knockin mice as a pre-symptomatic model that exhibits key DM1 molecular features but does not develop the severe symptoms expected for a full mutation allele. Studies in pre-symptomatic DM1 mice revealed novel susceptible cells/tissues which we propose are responsible for key aspects of DM1 symptoms. Based on the hypothesis that human sequences flanking the CTG site and/or longer expansions are required to fully model DM1 in mice, we have developed a new strategy to generate large repeat expansions for genome editing and to replace the mouse 3’UTR with the corresponding human sequence (hDMPK CTGexp) carrying a series of 12 to 1682 CTG repeats. We tested the method for 3’UTR replacement with 12CTG in mice and generated a control line that demonstrates the effect of human 3’UTR on Dmpk mRNA levels and translation. HDR templates for hDMPK 3’UTR CTGexp are being used to generated Dmpk h3’UTR CTGexp KI mice to assess DM1-relevant mis-splicing, disease pathomechanisms, and multi-systemic phenotypes. Humanized DMPK CTGexp KI mice will serve as disease models and in vivo therapeutic testing platforms.