SATB1 and p21 expression in CD30⁺LPD clinical samples. (A) Immunohistochemistry shows diffuse p21 expression in SATB1^– PCALCLs (PCALCL-1), whereas in SATB1⁺ PCALCLs (PCALCL-2), p21⁺ cells were <20%. (B) Double staining on LyP samples shows that, among large atypical lymphocytes, SATB1 (red) and p21 (green) detect distinct populations of atypical cells. Only a small portion of SATB1^– reactive lymphocytes was positive for p21 staining. DAPI (blue) was used to visualize cell nuclei.