Early stages in the development of the mouse embryo are characterized by rapid growth and dynamic changes in patterning and morphology. However, technical challenges in accessing and manipulating embryos after implantation in the uterus have hampered the identification of genes involved in these processes. Here I present the characterization of two mouse mutants, cetus and Mgrn1md-nc. Together these studies have uncovered novel roles for genes in regulating early development and patterning of the mouse embryo. cetus mutants were isolated from a forward genetic screen for recessive mutations which disrupt the overall morphology of the developing embryo. cetus embryos are small with defects in closure of the neural epithelium and a severe reduction in somitic mesoderm. Interestingly, positional cloning of cetus revealed a novel point mutation in helicase motif V of the DEAD/Hbox helicase, Ddx11. I found that the cetus mutation in Ddx11 results in widespread apoptosis at early embryonic stages without disrupting proliferation. These studies identified novel, tissuespecific requirements for DDX11 during mouse development and show that helicase motif V is essential for these processes. Mgrn1md-nc is a spontaneous, semi-lethal mutation that results in loss of MGRN1 (Mahogunin RING-finger 1). While the effects of loss of MGRN1 on pigmentation are well studied in Mgrn1md-nc mutant mice, the developmental defects in these mice have not been well characterized. I found that loss of MGRN1 results in the mis-expression of nodal target genes controlling left-right patterning including Lefty1, Lefty2 and Pitx2, resulting in congenital heart defects and death in ~50% of embryos. My characterization of Mgrn1 mutants uncovered a role for this ubiquitin ligase in the regulation of Nodal signaling and left-right patterning. To this date, MGRN1 remains the only ubiquitin ligase to have been identified with a direct role in leftright patterning.