The present invention features an isolated nucleic acid that includes a mutant DNA encoding a Candida rugosa lipase, wherein the mutant DNA is 80% identical to a wild-type DNA encoding the Candida rugosa lipase, and includes at least 12 (e.g., 13, 15, 17, or all) universal serine codons corresponding to CTG codons in the wild-type DNA. Each of the universal serine codons, independently, is TCT, TCC, TCA, TCG, AGT, or AGC. The Candida rugosa lipase can be Candida rugosa lipase 1, 2, 3, 4, 5, or 8.