Children with SLOS have autism spectrum disorders and the most severely affected have birth defects, such as cleft palates, learning disabilities and heart defects.

BBC: 'No-one had heard of my boys' syndrome'

The discovery that both boys had a rare metabolic condition called Smith-Lemli-Opitz syndrome (SLOS), which results from a failure of the body to make its own cholesterol, explained many of their struggles during their early years.

BBC: 'No-one had heard of my boys' syndrome'

Dr Lloyd-Evans explains that the absence of cholesterol in the brains of SLOS sufferers means that the neurons and the central nervous system are affected, causing anything from mild personality disorders to severe malformations and multi-organ disease.

BBC: 'No-one had heard of my boys' syndrome'