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ABSTRACT: Background. Accurate histopathological diagnosis of certain melanocytic skin lesions as benign or malignant can be notoriously difficult. Recently, four-colour fluorescence in situ hybridization (FISH) has emerged as an important tool for classifying these lesions. Aim. To evaluate the sensitivity and specificity of a melanoma FISH probe kit for accurate diagnosis of melanocytic tumours, and to validate its use with imprint-cytology specimens from the cut surface of tumours. Methods. In total, 50 melanocytic skin lesions (31 malignant melanomas, 10 benign melanocytic naevi, and 9 histologically challenging benign melanocytic skin lesions) were evaluated. The samples comprise 47 tissue specimens embedded in paraffin wax, and three imprint-cytology specimens from the cut surface of melanomas. FISH was performed using four locus-specific identifier probes [Ras responsive element binding protein (RREB)1, myeloblastosis viral oncogene homologue (MYB), cyclin (CCN)D1 and centromere of chromosome (CEP)6], and results were compared with the clinical long-term follow-up and histopathological diagnosis data. Results. The melanoma FISH probe distinguished between naevi and melanomas with a sensitivity of 100% and a specificity of 94.1%. The most sensitive criterion was a gain in 6p25 (RREB1), seen in 100% of cases, followed by CEP6-related MYB loss (48.1%), CCND1 gain (37%) and MYB gain (22.2%). More than three-quarters (77.8%) of melanomas were positive for two or more criteria. Positive FISH results were also obtained for the imprint-cytology specimens. Conclusions. FISH is a valuable diagnostic tool for differentiating between benign and malignant melanocytic lesions, providing a high degree of sensitivity and specificity. The probes displayed exceptional discriminative capacity in difficult or ambiguous lesions. To our knowledge, his is the first reported use of imprint-cytology specimens for FISH diagnosis.