When the gut barrier is impaired, intestinal Gram-negative bacteria release lipopolysaccharides (LPS) into the bloodstream, causing Toll-like receptor 4 (TLR4)-mediated endotoxemia [18,19], which induces pulmonary perivascular inflammatory responses and thus promotes the development of PAH. The gene discussed is TLR4; the disease is serum lipopolysaccharide activity.