Abstract Body: Congenital heart defects (CHDs) are be caused by mutations in genes that drive cardiac development, such as Tbx5, Gata4, and Mef2c. Cardiac development, and its transcriptional regulators are also regulated by microRNAs (miRs). These small RNAs target the transcripts of genes in numerous cardiac cell types during embryonic development. We and others have shown that the miR-200 family modulates the transcripts of cardiogenic transcription factors (TFs) Tbx5, Gata4, and Mef3c. However, the relationship between these miRs and cardiogenic TFs during in vivo cardiac development is poorly understood. During cardiogenesis, miR-200 family members are highly expressed (E14.5) but reduced in adults (3mo) (Ct Value: miR-200a: 24.3 ± 0.59 v 38.3 ± 0.21; miR-200c: 25.0 ± 0.64 v 32.5 ± 0.16). Using our miR-200 family inhibitor mice models (PMIS), we have found these miRs are required for cardiac development. PMIS-miR-200 embryos are found with a ventral septal defect and poor ventricle wall development, which is lethal by e16.5. At e14.5, PMIS-miR-200 hearts have a significant increase in expression of Tbx5, Gata4, and Mef2c compared to Wild-Type. This induced expression of these TFs is seen within CMs of the ventricle at E14.5. snMulti-Omics of WT and PMIS-miR-200 hearts found a population of CMs enriched in the PMIS-miR-200 hearts. These CMs are marked by expression of Tbx5, Nppa, and Sox5. RNA velocity analysis found these CMs to be “progenitor-like” and associated with an early pseudotime. Expression of Tbx5, Nppa, and Sox5 correlated along the pseudotime with the “progenitor-like” CMs. ATAC-seq showed enrichment of Tbx5 and Mef2c motifs within this new CM cell state. Conclusions: The miR-200 family is a modulator of cardiogenic TFs expression and activity during development. Inhibition of miR-200 induces a CM cell state with “progenitor-like” qualities. Future directions will determine the role of miR-200 in adult cardiac disease, such as ischemic injury. Our work provides new insights into gene dosage, modulated by miRs, that is required and necessary during cardiac development.