A genomic clone carrying a proteinase inhibitor Ⅰ sequence was isolated and characterized. The clone contained a 0.7 kb EcoRI fragment hybridized with tomato inhibitor Ⅰ cDNA. The nucleotide sequence of the EcoRI fragment revealed presence of a truncated form of a proteinase inhibitor Ⅰ gene of potato. The truncated gene contained the 5` flanking region and the first exon of a functional proteinase inhibitor Ⅰ gene. Although the 5` flanking region contained the regulatory sequences TATAAA and CCACT, a deletion of 40 bp occurred between them.