Congenital heart defects (CHDs) are the most common birth defect, making them a significant burden on the healthcare system and traumatic for the families affected. Defects in septation of the chambers and formation of the valves comprise the highest incidence of CHD subtype. Septation of the chambers is important to separate oxygenated and de-oxygenated blood, and in conjunction with development of the valves is critical for the heart to maintain unidirectional blood flow. Valves develop from the endocardial cushions of the atrioventricular canal (AVC) and the outflow tract (OFT), through a thickening of extracellular matrix (cardiac jelly) lying between the myocardial and endocardial cell layers. Endocardial cells can undergo a cell transformation, reminiscent of epithelial-mesenchymal transition (EMT), and invade the underlying cardiac jelly, thereby populating the ECM with mesenchymal cells required for valve development. Transmembrane protein 2 (Tmem2) was previously identified in a forward genetic screen in zebrafish to have a heart looping defect. Specifically, the wickham (wkm) mutants displayed an expansion of the AVC. Recent work has identified a role for Tmem2 as a hyaluronidase, breaking down long hyaluronic acid polymers. Using a conditional knockout for Tmem2 we hope to uncover the role that modifying hyaluronic acid plays in the ECM of the developing mouse heart valves. Using the Tmem2 knockout first allele, which expresses LacZ, we have identified Tmem2 expression in the endocardial cells of the developing heart at E9.5 and E10.5, via LacZ staining. Homozygous knockout embryos are not viable after E7.5. Our preliminary data from conditional knockout of Tmem2 in the endocardium, using a VE-cadherin-Cre, show significant changes to endocardial cushion morphology, consistent with a role for Tmem2 in early valve development.