However, there are lots of potential complications that hinder the long-term success of HSCT, in which bronchiolitis obliterans syndrome (BOS) is one of such serious complications. Basically, BOS represents a form of graft-versus-host immunological damage of small airways (bronchioles), leading to progressive narrowing of small airways and thus obstructive lung function abnormalities. With progressive loss of lung function in BOS, patients after HSCT can be complicated by intractable respiratory failure that results in mortality. Up until now, there is still no reliable way to accurately predict or detect BOS early to allow pharmacological interventions.