OBJECTIVE: Heritability of stroke is assumed not to be low, especially in the young-age stroke population. However, most genetic studies have been performed in highly selected patients with typical clinical or neuroimaging characteristics. We investigated the prevalence of 15 Mendelian stroke genes and explored the relationships between variants and the clinical and neuroimaging characteristics in a large, unselected, young stroke population. METHODS: We enrolled patients aged ≤55 years with stroke or transient ischemic attack from a prospective, nationwide, multicenter stroke registry. We identified clinically relevant genetic variants (CRGV) in 15 Mendelian stroke genes (GLA, NOTCH3, HTRA1, RNF213, ACVRL1, ENG, CBS, TREX1, ABCC6, COL4A1, FBN1, NF1, COL3A1, MT-TL1, and APP) using a customized, targeted next-generation sequencing panel. RESULTS: Among 1,033 patients, 131 (12.7%) had 28 CRGV, most frequently in RNF213 (n=59), followed by ABCC6 (n=53) and NOTCH3 (n=15). The frequency of CRGV differed by ischemic stroke subtypes (p<0.01)-highest in other determined etiology (20.1%), followed by large artery atherosclerosis (13.6%). It also differed between patients aged ≤35 years and those aged 51-55 years (17.1% vs. 9.3%, p=0.02). Only 27.1% and 26.7% of patients with RNF213 and NOTCH3 variants had typical neuroimaging features of the corresponding disorders, respectively. Variants of uncertain significance (VUS) were found in 15.4% patients. INTERPRETATION: CRGV in 15 Mendelian stroke genes may not be uncommon in the young stroke population. The majority of patients with CRGV did not have typical features of the corresponding monogenic disorders. Clinical implications of having CRGV or VUS should be explored. This article is protected by copyright. All rights reserved.< Réduire