Around 80% of rare diseases have identified genetic origins; they result from abnormalities in the genes or chromosomes, and are often described as congenital diseases. Such genetic defects can be inherited or caused by sporadic mutation.

The human body has close to 35,000 genes, of which around 5 to 10 are normally defective. If both parents carry the same defective gene, the risks are much higher that their offspring will inherit an active, dominant form of a particular disease and show its physical effects. Hence, the risk of an infant being born with rare disease exists in all pregnancies, at a rate of approximately 3 to 5%.1

Most of the rare diseases that patients diagnosed to date in Hong Kong suffer from are hereditary metabolic diseases.2

Another 20% of rare diseases are the result of infections (bacterial or viral), allergies and environmental causes, are degenerative and proliferative, or have causes that still remain unknown.

Some of the better known orphan diseases include Mitochondrial diseases, Mucopolysaccharidoses, Phenylketonuria, urea cycle diseases etc.


1. Source: Taiwan Foundation for Rare Disorders.
2. Source: Hong Kong Society of Inborn Errors of Metabolism.