A fruit-specific and pathogenesis-related 5/thaumatin-like (PR5/TL), 31-kDa protein was isolated by 2D-PAGE from fully-grown apples (Malus domestica cv. Fuji) and named Mdtl1 (Malus domestica thaumatin-like protein 1). Using the N-terminal sequence of the protein, the full-length cDNA encoding Mdtll was isolated. The cDNA clone (Mdtl1) consists of 944 bp with an open reading frame (ORF) of 744 bp encoding a protein of 247 amino acids. The deduced amino acid sequence of Mdtl1 shows high similarity to the sequences of PR5/TL proteins. Mdtl1 is a slightly acidic protein with a putative signal peptide and a putative N-glycosylation site, and lacks a C-terminal extension. This suggests that Mdtl1 is an apoplastic glycoprotein. Results of northern blotting indicated that expressions of Mdtl1 are developmentally regulated. Southern blot analysis showed that Mdtl1 may be present as a single copy, and there exist other genes closely related to Mdtl1 in the apple genome.