The research work in the book,demonstrated successful cloning of a partial ∆6 desaturase gene from Pangasianodon hypophthalmus and investigated the HUFA biosynthetic activity in it. The results on the basis of liver fatty acid composition and gene expression study by real time PCR suggest that P. hypophthalmus may convert linolenic acid into HUFA. These results were confirmed on the molecular level, which demonstrated that the transcriptional control of ∆6 desaturase gene expression is one mechaninsm whereby dietary fatty acids influence the activity of HUFA biosynthetic pathway. The results can provide direction for the selection of lipid source or determine the substituted ratio of fish oil by vegetable oils in the diets for freshwater fish such as P.hypophthalmus