The reason for a once-promising class of drugs for being unable to help people who are suffering from multiple sclerosis has been found out.

This is due to a genetic variant that has been linked to MS and according to an Oxford University team, due to this the drugs which work for patients with other autoimmune diseases will not work for them.

The team also added that things can become worse with this drug.

The response to treatment is affected by a person's genetic make-up, experts said.

The drugs have not been able to help people with MS but they have been able to help patients with rheumatoid arthritis and inflammatory bowel disease.

TNFRSF1A is a gene that has one particular genetic variant and the Oxford University team looked at that and previously this has been linked to risk of developing MS.

TNF signaling molecule is essential for many processes in the body and the normal, long version of the protein sits on the surface of cells and binds the TNF.

Professor Lars Fugger of the Nuffield Department of Clinical Neurosciences, who led the work, said, "The hope has been that analyses of the whole human genome would lead to findings that are clinically relevant."

But the team discovered that the signals were not triggered as the variant caused the production of an altered, shortened version which mops up TNF and this is exactly what the TNF blocking drugs do.