[music plays]
[NARRATOR:] A patient with cystic fibrosis
is susceptible to infections of the lungs.
This is due to a mutation that affects a protein
on the surface of lung cells.
Many different mutations can cause cystic
fibrosis, but all of them involve a transmembrane
protein called CFTR that functions as a channel
for chloride to move in and out of cells.
In this case, CFTR is less able to transfer
chloride ions out of the cell,
compared to the functioning protein 
in people without cystic fibrosis.
The channel is, in effect, locked shut.
This inability to transfer chloride ions has
an effect on the outside of the cell.
In people without cystic fibrosis, there is
a balance of chloride ions inside and outside
lung cells, which maintains the right amount
of water and salt ions
in the mucus lining the airways.
Cilia normally sweep the watery mucus from
the lung lining, keeping the surface clean.
In patients with cystic fibrosis, chloride
ions concentrate inside cells
and draw in water by osmosis.
With less water, the mucus becomes thicker
and cilia can’t move.
Flow is reduced, providing an environment
for bacterial infection.
But there are now drugs that can help patients.
To find a drug, researchers tested over 200,000
small molecules to see if any would bind to
the CFTR protein.
Then, for each small molecule that had some
binding ability, researchers chemically synthesized
different variations of that molecule and
tested them for the ability to improve the
function of some types of mutant CFTR channels.
They identified a drug that, when taken by
patients with a certain type of CFTR mutation,
binds to the CFTR protein, opening the channel
and allowing improved ion transfer.
This increases chloride ion flow and maintains
a watery, clear mucus, which helps the cilia
move more freely and reduces the risk of bacterial
infection.
