Apela (also referred to as Elabela, Ende and Toddler) is a small signaling peptide that activates the G protein-coupled receptor Aplnr. We used CRISPR/Cas9 to generate a null, reporter-expressing allele, in order to study the role of Apela in the developing mouse embryo. We found that loss of Apela results in low penetrance cardiovascular defects that manifest after the onset of circulation. Targeted Apela null alleles exhibited different transcriptional activity depending on the presence or absence of a Neomycin selection cassette. These are referred to as Apela KO NEO-IN and Apela KO NEO-OUT strains, respectively. Despite subtle phenotypic characteristics that were unique to the NEO-OUT mutants, both Apela null strains shared the same variable expressivity of cardiovascular defects and the same penetrance of embryonic lethality. To investigate the earliest regulatory events leading to physical abnormalities in Apela mutants, we performed RNA-Seq on whole stage-matched and morphologically normal E7.5 embryos (3 wild-type, 6 Apela KO NEO-IN, and 6 Apela KO NEO-OUT individuals). We chose this stage because Apela is initially expressed in the embryo at late gastrulation, shortly after the emergence of extraembryonic mesoderm progenitors. Since modification of the Apela locus may influence the expression of neighboring genes, we examined the expression of upstream and downstream sequences and found no significant difference in their expression. Downregulated genes of interest included several mitochondrial genes, Ceacam2, Ulk4, and Mov10l1. Upregulated genes included the vascular endothelial growth factor Vegfc. Principal component analysis identified outliers (KO1 and KO9), both of which expressed lower levels of mesoderm markers. KO9 was further characterized by aberrant upregulation of erythroid and myeloid markers. This finding was confirmed in our study by qRT-PCR analysis of additional Apela null individuals. Overall design: 15 individual embryos were analyzed at E7.5. Embryos were stage-matched according to morphological landmarks. Control samples were wild-type (n=3), and Apela KO samples were null embryos from the NEO-IN (n=6, ‘KO1-6’) and NEO-OUT (n=6, ‘KO7-12) mutant strains. Whole embryos (including embryonic and extraembryonic tissues) were used for the analysis. Apela KO samples were isolated from homozygous KO intercrosses and therefore did not require genotyping.