Nucleolar organizer regions in Spitz nevi and malignant melanomas.

Nucleolar organizer regions (NOR) are loops of DNA that transcribe ribosomal RNA; they can be easily identified in formalin fixed paraffin embedded tissue using a silver (Ag) method. It has been suggested that the number of AgNOR per cell can differentiate between benign and malignant melanocytic lesions of skin. We have studied 29 Spitz nevi (SN) and 39 invasive malignant melanomas (MM) by the same silver method. SN showed between 1.0 and 1.6 AgNOR per cell with a mean of 1.2. MM counts ranged from 1.2 to 4.2 with a mean of 2.0. It is concluded that the AgNOR method cannot reliably differentiate SN from MM; however, a count of more than 2.0 AgNOR per cell would favor a diagnosis of MM rather than SN.