But in cystic fibrosis, a defective gene causes the secretions to become thick and sticky.
Men have one X and one Y, so a defective gene on the X shows itself readily.
One of them targets a defective gene on the seventh chromosome, present in roughly 7% of all cancers, including most cases of melanoma.
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).
But a defective ATM gene can interfere with p53 thereby allowing potentially cancerous cells to multiply.
After years of controversy, gene therapy is poised to become a viable option for a variety of often life-threatening medical conditions, especially those resulting from a single defective gene.
These orexin cells were discovered from the study of narcolepsy, a disease in which people cannot stay awake, in the late 1990s by Emmanuel Mignot, a Stanford researcher, who found that dogs with a defective orexin receptor gene developed the condition.
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The reason kids like Arya live at all is that humans have a slightly defective cousin of the SMA gene that produces only enough nourishing protein to keep patients alive for a while.
It could be a gene that, defective in some way, predisposes a person to disease.
If it is safe, the medicine could also be used for patients who have a single copy of the defective gene.
Each of their children has a 50% chance of inheriting the defective gene.
But the nearest-term potential for the drug will be in patients who have a rare genetic disease in which both copies of a gene that controls getting cholesterol out of the body are defective.
Women have two X chromosomes, and a normal copy of the gene will override the defective one.
In addition to Tarceva and Avastin, it sells the breast cancer drug Herceptin, which targets a gene called Her-2 that is defective in about 25% of breast cancer patients.
But its genetic origins remained shrouded in mystery until 1995, when French researchers pinpointed a gene on chromosome 5 that was missing or defective in 98% of those afflicted.
In one trial of 290 patients who were given Glivec for at least six months, 56% enjoyed a powerful response: The drug destroyed most or all of the marrow cells with the defective gene.
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.
And a brand new kind of software allowed him to use information about the family and their genetic material to find the defective gene.
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