MMP21 and Dextrocardia: In summary, our findings show that two de novo, heterozygous non-synonymous MMP21 variants p.G244E and p.K487E affect the establishment of left–right asymmetry in zebrafish as well as play an indispensable role in heart looping, supporting the causal effect of heterozygous MMP21 variants in the dextrocardia phenotype found in patients with HTX and CHDs.