A review.To classify a disease as allelic to another genetic disease caused by mutations in the same gene, the symptoms need to be distinct and non-overlapping, not demonstrate varying severity.Both Papillon-Fefevre syndrome and Haim Munk syndrome are caused by mutations in the gene coding for cathepsin C.Their phenotypes overlap to include PPK and periodontitis, but in Haim Munk syndrome, arachnodactyly, acroosteolysis, and onychogryphosis also develop.Because of this, HMS is considered a distinct entity allelic to PLS.Clearly, the situation for PPK-GN and MDM is quite different.These two conditions represent a spectrum of disease severity, as is seen in most genetic disorders.We consider that PPK-GN represents only the mild end of the MDM spectrum.We propose that the classification of PPK-GN as a sep. genodematosis should be abandoned.