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
Treatment for a progressive disease that destroys red blood cells and a second condition that damages the kidney and other vital organs.
Company: Alexion Pharmaceuticals (U.S.)
Forecast 2012 sales: $1.1 billion.

Cerezyme
Helps break down fatty clumps that build up in cells in patients with Gaucher disease, damaging the liver, spleen and bones.
Company: Sanofi (France)
Forecast 2012 sales: $830 million

Myozyme
Treatment for Pompe disease, an inherited neuromuscular disorder that causes progressive muscle weakness.
Company: Sanofi (France)
Forecast 2012 sales: $580 million

Elaprase
Treatment for Hunter syndrome, a genetic disease that primarily affects males, causing serious physical and mental problems.
Company: Shire PLC (U.K.)
Forecast 2012 sales: $495 million

Fabrazyme
Treatment for Fabry disease; designed to counter damaging symptoms including kidney failure, heart problems and stroke.
Company: Sanofi (France)
Forecast 2012 sales: $352 million

Cinryze
Prevents dangerous swelling and painful attacks in people with hereditary angioedema.
Company: ViroPharma Inc. (U.S.)
Forecast 2012 sales: $330 million

Vpriv
Treatment for Gaucher disease.
Company: Shire PLC (U.K.)
Forecast 2012 sales: $310 million

Kalydeco
Treats a rare form of the lung disease cystic fibrosis in patients with a particular genetic mutation.
Company: Vertex Pharmaceuticals Inc. (U.S.)
Forecast 2012 sales: $260 million

Naglazyme
Treatment for Maroteaux-Lamy syndrome, which causes short stature, stiff joints and breathing problems.
Company: Biomarin Pharmaceutical Inc. (U.S.)
Forecast 2012 sales: $233 million

Source: Sales figures based on analyst forecasts compiled by Thomson Reuters