Additionally, three rare heterozygous non-synonymous MMP21 variants were identified in three unrelated Chinese Han patients with HTX and complex cardiac malformations: p.G244E in dextrocardia, p.L277F in dextrocardia and RAI, and p.K487E in RAI (Table 2; Additional file 2: Table S2). This evidence concerns the gene MMP21 and right atrial isomerism.