What is MHE-MO?

Multiple Hereditary Exostoses/Multiple Osteochondromas (MHE-MO) is a disease that affects the long bones (e.g. arms and legs) of the skeleton, the pelvis, ribs and shoulder blades. It is characterized by a large number of benign bone tumours (a.k.a. exostoses/osteochondromas). They have a cartilage top and sometimes present with a bursa.

The osteochondromas often develop at a very young age and grow, mainly during the growth time of that person. MHE-MO is a hereditary congenital disease and has a autosomal dominant inheritance pattern – meaning one only needs one mutated gene to be born with the disease. This also means that MHE-MO patients have a 50% chance to pass on the disease to their children. In 38% of cases, one is born with MHE-MO as a result of a so-called spontaneous mutation – neither one of the parents has MHE-MO. Because of the autosomal dominant inheritance pattern, one cannot be a carrier of the disease.

MHE-MO is caused by one of two known genes: EXT1 or EXT2 on chromosome 8 and 11. MHE-MO is a rare disease: it affects about 1 in 50.000 people, according to the scientific literature. It is estimated, that there are about 1000 individuals with MHE-MO in the Netherlands.


The World Health Organization (WHO) changed the name of MHE to Multiple Osteochondromas (MO), because the term “exostoses”, meaning bone outgrowth, is considered confusing and unspecific. According to WHO, a diagnosis of MO can be made, when at least two osteochondromas have been found on a patient. Besides that, there must be at least one family member with MO or with a mutation in one the EXT-genes.

In Dutch we use the name ‘’Multiple Osteochrondomen’’. Within the HME-MO Association both names, ’’Hereditaire Multiple Exostosen’’ and ‘’Multiple Osteochondromen’’ (HME-MO), are still being used. The diagnosis of MHE-MO is relatively easy to make, mainly based on x-rays. Sometimes additional examination is required, with the help of MRI scans. An orthopaedic surgeon may ask for genetic testing in order to confirm the diagnosis. The specialist may also ask the Bone Tumour Committee of the Netherlands for advice, without any costs involved.