A study conducted by Sobanski et al. compared the clinical characteristics of CTD-PAH patients with and without anti-U1 RNP antibodies: anti-U1 RNP-positive patients had younger ages, shorter CTD duration, and milder functional impairments (lower WHO functional class, longer 6-min walk distance, higher diffusion capacity for carbon monoxide) when PAH was diagnosed. Here, U1 is linked to pulmonary arterial hypertension.