CD37 is expressed by Richter syndrome primary samples and patient-derived xenograft models. CD37 expression on primary RS samples was checked by immunohistochemistry performed on bone marrow or lymph node biopsy specimens from patients with RS (yellow dots) or paired CLL-RS samples (A; red-green-dark blue-purple dots). Representative staining of 2 paired CLL-RS and 3 RS samples. Original magnification, ×200; inset ×6. CD37 expression was checked, both at the transcript and protein levels, in our 4 RS-PDX models (RS1316, RS9737, RS1050, and IP867/17) by RNA sequencing (B; n = 4 per model; box and whiskers, minimum to maximum representation), quantitative real-time PCR (C; n = 8 per model; box and whiskers, minimum to maximum representation), western blot analysis (D; n = 4 per model; box and whiskers, minimum to maximum representation) and immunohistochemistry staining on slides of formalin-fixed and paraffin-embedded, RS-PDX–derived tumor masses (E). In western blot analyses, actin was used as a loading control. RE, relative expression; TPM, transcript per million.