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||P.1 left column top paragraph: "In humans, ∼35% of all genes have introns in their 5′ untranslated regions (UTRs) [primary sources]. These introns differ from those in coding regions, for example, in typical length and nucleotide composition [primary sources]. Previously, 5′UTR introns (5UIs) were suggested to be evolving under a neutral model of random insertion and deletion events with the sole constraint of avoiding upstream open reading frames [primary source 3]. Recently, [investigators] showed that presence and length of 5UIs correlates with the level of expression across cells and tissue types [primary source 1]. More importantly, [they] observed an uneven distribution of 5UIs amongst genes across specific functional categories [primary source 1]."