Skin melanoma is a common form of skin cancer. Its diagnosis is usually clinically suspected and then affirmed by histopathological examination.

In some cases, the histopathological analysis of these lesions is equivocal, and the malignant or benign nature of the proliferation is difficult to determine with certainty. In these cases, the use of expensive ancillary tests, which are hardly accessible and take a long time to set up (FISH - CGH-array), is usually required.

Anti-PRAME immunostaining, an inexpensive and readily available technic, has recently been described as highly sensitive and specific for diagnosing malignant melanocytic proliferations. Knowledge on its utility for evaluating ambiguous melanocytic neoplasms remains limited.

Our study aims to improve the current body of knowledge on the utility of PRAME immunohistochemistry for evaluating challenging samples of melanocytic proliferation. The secondary objective is to determine the PRAME immunoreactivity profile for each histological subtype of melanocytic proliferations (spitzoid tumors, cellular blue nevi, dysplastic nevi ...)

Inclusion Criteria:

• Adult patient at the time of diagnosis (> = 18 years).
• histopathological diagnosis of "ambiguous melanocytic proliferation" or "atypical melanocytic proliferation" made between January 2016 and December 2020.
• Histological slides archived in the "CRB" biobank available
• Formalin-fixed paraffin-embedded tissue block archived in the "CRB" biobank available
• Informed patient

Exclusion Criteria:

• Patient who has expressed opposition to participating in the study.

All cases with histopathological diagnosis of "ambiguous melanocytic proliferation" or "atypical melanocytic proliferation" made between January 2016 and December 2020.