Be aware of aberrant reactivity in the tumor (unexpected positive or negative staining) especially when limited panel of antibodies is used.

A) Aberrant reactivity for CD20 is a rare event and may be confusing.

B) Malignant melanoma may relatively often especially in metastatic setting expresses aberrant reactivity for different antibodies inluding rare aberrant positive staining for CD45.

C) Malignant melanoma especially in metastatic setting may be non reactive for its typical markers (S-100, Melan A, HMB45) and may show reactivity with markers characteristic for another lineage of differentiation (transdifferentiation). In such cases the clinical data are essential for right diagnosis.

D) Sometimes aberrant reactivity may surprisingly dominate the immunoprofile leading to erroneous conclusions.

E) Another example of aberrant staining in metastatic malignant melanoma.

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