Germline mutations in the STK11/LKB1 gene on 19p13.3 are found in 30–70% of PJS cases, depending on the screening method, with considerable uncharacterised genetic heterogeneity remaining in this syndrome.3,4 The disease causing gene has been identified by two independent groups.5,6 Human STK11 encodes a serine/threonine protein kinase that is highly homologous to the mouse protein Lkb1 and the Xenopus kinase XEEK1,7,8 and is expressed in all human tissues.9 The kinase domain of the human 433 amino acid protein is localised between residues 49 and 309,7 and shows homology to the conserved catalytic core of the kinase domain common to both serine/threonine and tyrosine protein kinase family members.10 Most mutations found in PJS patients are small deletions/insertions or single base substitutions leading to an abnormal truncated/kinase inactive protein.