Carboxyl ester lipase (CEL) encodes a cholesterol ester hydrolase that is secreted into the duodenum as a component of pancreatic juice. The objective of this study was to characterize the CEL gene, investigate the association between the CEL promoter variants and chicken phenotypic traits, and explore the CEL gene regulatory mechanism. An insertion/deletion (indel) caused by a 99-bp insertion fragment was shown for the first time in the chicken CEL promoter, and large differences in allelic frequency were found among commercial breeds, indigenous and feral birds. Association analysis demonstrated that this indel site had significant effects on shank length, shank girth, chest breadth at 8 weeks (p < 0.01), evisceration weight, sebum weight, breast muscle weight, and leg weight (p < 0.05). Tissue expression profiles showed extremely high levels of the CEL gene in pancreatic tissue. Moreover, the expression levels of the genes APOB, MTTP, APOV1 and SREBF1, which are involved in lipid transport, were significantly reduced by adding a 4% oxidized soybean oil diet treatment at the individual level and transfecting the embryonic primary hepatocytes with a CEL-overexpression vector. Interestingly, the results showed that the expression level of the II homozygous genotype was significantly higher than that of the ID and DD genotypes, while individuals with DD genotypes had higher phenotypic values. Therefore, these data suggested that the CEL gene might affect body growth by participating in hepatic lipoprotein metabolism and that the 99-bp indel polymorphism could be a potentially useful genetic marker for improving the economically important traits of chickens.Carboxyl ester lipase (CEL), also known as cholesterol esterase or bile salt-dependent lipase, is attributed to its ability to hydrolyze multiple lipids and is a lipolytic enzyme with broad substrate specificity 1,2 . The enzyme is primarily synthesized and expressed in large quantities in lactating mammary glands and in the exocrine pancreas and is stored in zymogen granules, which are then activated by bile salts in the intestine 3-5 . After consumed feed reaches the gastrointestinal tract, the enzyme is released into the intestine, mainly as a supplement to other lipolytic enzymes. Although it constitutes only 1-5% of the pancreatic enzyme content, it has an important effect on the hydrolysis of lipid nutrients in dietary fat 6-8 . In addition, CEL secreted by the liver and present in plasma not only contributes to the assembly and secretion of chylomicrons but also participates in low-density lipoprotein (LDL) lipid metabolism, the selective uptake of cholesterol esters and reverse cholesterol transport in high-density lipoprotein (HDL) 9-11 .According to the National Center for Biotechnology Information (NCBI) database, the CEL gene contains 11 exonic sequences in human, bovine, murine, chicken and other vertebrate genomes. Interspecific comparisons of genes and proteins have shown that the CEL gene retains essentially similar properties, structures and key...