FASN and steatosis: Here, we use zebrafish genetics and biochemical analyses to show that Atf6 is both necessary and sufficient for steatosis by showing that: (i) Atf6 is required for alcoholic steatosis, (ii) activation of Atf6 is sufficient to cause steatosis, (iii) Atf6 activation induces expression of genes involved in glyceroneogenesis and fatty acid elongation and causes de novo lipogenesis, and (iv) Atf6 epistatically interacts with fatty acid synthase (fasn), a key enzyme involved in TAG biosynthesis, to cause FLD.