Walk-PHaSST study is a multi-center clinical trial to evaluate the effects of sildenafil on Sickle Cell Disease (SCD) population with low exercise capacity associated with an increased Doppler-estimated pulmonary artery systolic pressure (as assessed by the tricuspid regurgitant velocity (TRV)). It is a double-blind, placebo-controlled trial to assess the safety, tolerability, and efficacy of sildenafil in patients with SCD who had both an elevated TRV and decreased exercise capacity. The screening phase of the study enrolled 720 subjects and 74 of them were randomized for the clinical trial. For the screening cohort, we have collected general demographics and race/ethnicity, hemoglobin genotype, physical examination, laboratory screening, transthoracic Doppler echocardiography, and 6 minute walk test in 9 US and 1 UK site. The walk-PHaSST biorepository have banked samples from 610 of the screening subjects. A total of 592 subjects from the screening phase are included in the TOPMed program for WGS.