MYH11 and megacystis-microcolon-intestinal hypoperistalsis syndrome 1: While gain-of-function variants in <i>MYH11</i> are associated with thoracic aortic aneurysms and dissections, loss-of-function variants are linked to autosomal recessive Megacystis-Microcolon-Intestinal Hypoperistalsis Syndrome (MMIHS).