Abstract: A novel selenocysteine-containing protein is provided which is a substantial entity having an enzymatic activity, typically an activity to reduce peroxide phospholipids. There are provided a gene consisting of a coding sequence for a protein not containing selenocysteine in which the codon for selenocysteine is introduced at a desired position and a selenocysteine insertion sequence resided at the 3?-end of said coding sequence and a protein expressed therefrom, typically a selenocysteine-containing protein having an activity to reduce peroxide phospholipids which is prepared by introducing selenocysteine into albumin and as well as a gene encoding said protein. A novel anti-oxidative substance is provided which can be applied for prevention of worsening, prophylaxis or treatment of various diseases associated with oxidative stress.