TagA (3-methyladenine-DNA glycosylase I) excises 3-methyadenine and 3-methylguanine from alkylated DNA. The structure of this enzyme has not yet been determined experimentally. We propose a three-dimensional model of the TagA protein based on the threading algorithm. The model shows that TagA is a mostly alpha-helical protein, in agreement with circular dichroism measurements. None of the eight cysteines present in the TagA sequence forms a disulfide bridge in the model structure, which has also been experimentally verified with the use of Ellman method.