Functional analysis indicated that the underlying mechanism of HTX and CHD may not be the haploinsufficiency of MMP21, although loss of function in MMP21 can cause the phenotypes of situs inversus with CHD in mice with chemically induced missense mutations c.677T>C, p.(Ile226Thr) or p.(Ala321Pro) (Qin et al. 2022). This evidence concerns the gene ZIC3 and situs inversus.