CK9 can be detected in primary cultures of palmoplantar keratinocytes when they shift to differentiation-promoting conditions and grow stratified (upper cells). CK9 has not been found in normal, i.e. non-pathogenic, non-ridged epidermis, beside some minor cells surrounding the acrosyringeal ducts. No labelling has been found in epithelial cells of other stratified epithelia such as oe-sophagus or complex epithelia (e.g. urothelium) or in ductal or simple epithelia. Negative tissues include: muscle, liver and duodenum. (see references listed below).