<qa>  What is the PAH of SCD considered to be a rare disease in the US according2. SCD is considered to be a rare disease in the US according to the NIH. As PAH of SCD probably only occurs in 2-4 of HbSS adults, many centers in the US will only have a few 10 eligible patients for enrollment even under the best circumstances. SCD patients often have co-morbidities limiting their participation in clinical trials, which often decreases this number even further. 