Germline mutations of the LKB1 gene are associated with Peutz-Jeghers syndrome (PJS), which is characterized by mucocutaneous pigmentation and gastrointestinal hamartoma with an increased risk of cancer development. In this study, we have employed polymerase chain reaction and DNA sequencing analysis to characterize the LKB1 gene in a 25-year-old Japanese PJS patient. Direct sequence analyses revealed a novel single base deletion at nucleotide 844 in exon 6 (844delC) in one LKB1 allele, resulting in a frame shift and in the introduction of a premature termination codon in this mutated allele.