Germline mutations in the LKB1/STK11 serine/threonine kinase cause Peutz-Jeghers syndrome and this gene is also mutated at a moderate frequency in a wide variety of sporadic tumours. The translated region of LKB1/STK11 (1302bp) codes for a serine/threonine kinase of otherwise unknown function. We report a novel LKB1/STK11 mRNA species which is found at variable levels in all tissues examined. The novel mRNA, which we believe may be an unusual splice variant, consists of a 444bp in-frame deletion of exons 5-7 and part of exon 8. This deletion removes a large part of the kinase domain and comparison with other LKB1/STK11 mutations shows that kinase function is undoubtedly abolished. The role of the novel mRNA species remains unclear, but it retains a putative cAMP-dependent kinase phosphorylation site and may play some regulatory role.