-galactosidase enzyme will be produced in a cell in which non-sense mutation takes place in the lac y gene. Non-sense mutation stops polypeptide synthesis-due to the formation of non-sense codon. In lac operon, sequence of structural genes is z (codes for -galactosidase), y (permease) and a gene (transacetylase). Due to non-sense mutation at y-gene, permease synthesis will be stopped resulting in non-expression of both y and successive gene a also. Thus, only -galactosidase enzyme will be produced.