Rare diseases are a growing focus for drug companies, with prices for some treatments for ultra-rare conditions running into hundreds of thousands of dollars a year.

The following are among the top-selling treatments for extremely rare disorders:

Soliris, Alexion Pharmaceuticals

- Treatment for a progressive disease that destroys red blood cells and a second condition that damages the kidney and other vital organs.

Forecast 2012 sales: $1.1 billion.

Cerezyme, Sanofi

- Drug to help break down fatty clumps that build up in cells in patients with Gaucher disease, damaging the liver, spleen and bones.

Forecast 2012 sales: $830 million

Myozyme, Sanofi

- Treats Pompe disease, an inherited neuromuscular disorder that causes progressive muscle weakness.

Forecast 2012 sales: $580 million

Elaprase, Shire

- Treatment for Hunter syndrome, a genetic disease that primarily affects males, causing serious physical and mental problems.

Forecast 2012 sales: $495 million

Fabrazyme, Sanofi

- Fabry disease treatment designed to counter damaging symptoms including kidney failure, heart problems and stroke.

Forecast 2012 sales: $352 million

Cinryze, ViroPharma

- Prevents dangerous swelling and painful attacks in people with hereditary angioedema.

Forecast 2012 sales: $330 million

Vpriv, Shire

- Treatment for Gaucher disease.

Forecast 2012 sales: $310 million

Kalydeco, Vertex Pharmaceuticals

- For treating a rare form of the lung disease cystic fibrosis in patients with a particular genetic mutation.

Forecast 2012 sales: $260 million

Naglazyme, Biomarin Pharmaceutical

- Treatment for Maroteaux-Lamy syndrome, which causes short stature, stiff joints and breathing problems.

Forecast 2012 sales: $233 million