A nomenclature system has recently been suggested for the description of changes (mutations and polymorphisms) in DNA and protein sequences. These nomenclature recommendations have now been largely accepted. However, current rules do not yet cover all types of mutations, nor do they cover more complex mutations. This document lists the existing recommendations and summarizes suggestions for the description of additional, more complex changes. Another version of this paper has been published in Hum Mut 15:7-12, 2000.