Some 600, 000 Americans have a less severe version caused by one defective copy. (There would be more of the double-gene victims if they didn't die young.) To get its drug approved Isis will need to study 45 patients with the severe two-gene form and a few hundred with the single-gene disease.

FORBES: Degunker

If it is safe, the medicine could also be used for patients who have a single copy of the defective gene.

FORBES

The idea of treating disease by replacing a defective gene with a working copy gained credence in 1990 with the success of the world's first gene therapy clinical tests against a rare condition called severe combined immunodeficiency (SCID).

MSN: European regulators back first gene therapy drug

Women have two X chromosomes, and a normal copy of the gene will override the defective one.

WSJ: New Outlook for Colorblindness