Annual cost: $409,500

Manufacturer: Alexion Pharmaceuticals

This drug treats paroxysmal nocturnal hemoglobinuria (PNH), an disorder in which immune system destroys red blood cells. Untreated, it is usually lethal within 10 years. It hits 8,000 people in the U.S.

Estimated annual cost: $375,000

Manufacturer: Shire

This treats something called Hunter syndrome (Mucopolysaccharidosis II, MPS II), which causes flattened noses, enlarged tongues, breathing difficulty and brain damage. Two thousand patients in the world have it.

Annual cost: $365,000

Manufacturer: BioMarin Pharmaceuticals

This treats a metabolic disorder called Maroteaux-Lamy Syndrome (MPS VI), which afflicts 1,100 people worldwide. Symptoms include enlarged head, thick lips and short stature. Patients may be wheelchair-bound by age 15.

Annual cost: $350,000

Manufacturer: ViroPharma

Cinryze treats hereditary angiodema (HAE), an immune system disorder that causes swelling in the abdomen and lips. The disease affects 6,000 people in the U.S.

Annual cost: $300,000

Manufacturers: Genzyme, Biomarin Pharmaceuticals

Myozyme replaces the enzyme that is missing in Pompe disease, a potentially fatal disease that weakens the muscles and enlarges the heart. Without treatment, late-onset patients eventually need wheelchairs and ultimately ventilators. Nine hundred people worldwide suffer from this.

Annual cost: $250,000

Manufacturer: Regeneron

Muckle-Wells, which this drug treats, hits 2,000 people worldwide and causes recurring fevers, rashes, joint pain and kidney damage.

Annual cost: $200,000

Manufacturer: Genzyme

Fabry disease causes burning sensations, purple blemishes, enlarged hearts and kidney problems. The number of patients who have it is 2,200 worldwide.

Annual cost: $200,000

Manufacturer: Genzyme

In Gaucher disease fatty clumps accumulate in spleen, liver, lungs, bone marrow and sometimes in the brain, causing skeletal disorder, lung and kidney impairment. The drug replaces the enzyme that Gaucher patients are missing. Worldwide, 5,200 patients have the disease.

Annual cost: $200,000

Manufacturer: Genzyme, Biomarin

This treats Hurler syndrome (MPS I), which causes patients to stop developing mentally and physically by age 4. Six hundred people in the world have it.