Histologic features of 2 representative patients with primary C-ALCL and a patient with tumor stage MF included as a positive control. The dermis of the first patient with C-ALCL (patient 10) shows a dense dermal infiltrate of CD30⁺ tumor cells with infiltration of the hair shaft (A). The tumor cells are predominantly negative for p63 (B). FISH with break-apart probes for TP63 (TP63 FISH) shows 2 normal fusion signals (C). The second patient with C-ALCL (patient 4) also shows a dense dermal infiltrate of CD30⁺ tumor cells (D). The tumor cells variably express p63 (E). TP63 FISH shows 3 extra copies of the fusion signal, indicating gain of chromosome 3 or polyploid tumor cells (F). The hematoxylin and eosin staining of the patient with MF shows some epidermotropism and diffuse infiltration of the dermis by blastic tumor cells (G). The tumor cells variably express p63 (H). TP63 FISH shows 2 fusion signals and 2 separate green signals (I; arrows), suggesting gain of chromosome 3 with an unbalanced translocation of TP63. Note that the epidermis shows expression of p63 in the basal and suprabasal layers (B,E,H). Original magnification ×200 for panels A-B,G-H, ×100 for panels D-E, and ×400 for insets.