Inhibitor of lipoprotein binding to the low density lipoprotein (LDL) receptor, LDL receptor-related protein, and very low density lipoprotein (VLDL) receptor. Associates with high density lipoproteins (HDL) and the triacylglycerol-rich lipoproteins in the plasma and makes up about 10% of the protein of the VLDL and 2% of that of HDL. Appears to interfere directly with fatty acid uptake and is also the major plasma inhibitor of cholesteryl ester transfer protein (CETP). Binds free fatty acids and reduces their intracellular esterification. Modulates the interaction of APOE with beta-migrating VLDL and inhibits binding of beta-VLDL to the LDL receptor-related protein. Synthesized mainly in liver and to a minor degree in intestine. Also found in the lung and spleen. Apolipoprotein C-I is present in acidic (APOC1A) and basic (APOC1B) forms in P.paniscus, P.abelii and P.troglodytes and perhaps also in baboons and macaques. The two genes for ApoC-I arose through a duplication process that occurred after the divergence of New World monkeys from the human lineage. In human, the acidic form has become a pseudogene sometime between the divergence of bonobos and chimpanzees from the human lineage and the appearance of the Denisovans. Pseudogenization resulted when the codon for the penultimate amino acid in the signal sequence was changed to a stop codon. Belongs to the apolipoprotein C1 family. Apolipoprotein C1 entry