Although basically bilaterally symmetrical, the body has various L–R asymmetries, particularly in the heart and viscera. The classic condition, situs inversus, is apparently due to a dynein defect but a number of other genes are known to be implicated in the specification of the L–R axis. These include nodal; lefty; rotatin, the transcription factor ptx2 and ZIC3. An autosomal form of visceral heterotaxy (HTX2) can be caused by mutation in the CFC1 gene that encodes the CRYPTIC protein involved in nodal signalling, or in the type IIB activin A receptor. There is also an X-linked form of the disorder.
Subjects: Medicine and Health.