To qualify for the experiment patients have to have experienced at least four vaso occlusive crises  the painful episodes associated with the disease  within a year of screening or at least two within the last six months. Study participants must also have previously received one or possibly two other sickle cell medicines. The more targeted focus could mean a far smaller market opportunity for Fulcrum which is already facing a large group of 10000 patients in the US fit the new study criteria a fraction of the roughly 100000 in the country believed to have the disease