ROR1 expression on primary RS samples. ROR1 expression was analyzed by IHC staining in primary samples of BM or lymph node biopsies from patients with RS using an anti-ROR1 antibody (mouse clone OTI3D11). Slides were scanned using an Aperio AT Turbo system to produce whole slide images. Slides were developed with DAB (3,3'‑diaminobenzidine) and counter-stained with hematoxylin and eosin. (A-C) A .jpeg image of each stain is shown. Original magnification ×20. An H score (range of 0-300) was calculated based on the summation of the product of the percentage of cells stained at each intensity using the following equation: (3 × % cells staining at 3+) + (2 × % cells staining at 2+) + (1 × % cells staining at 1+) to estimate ROR1 expression. ROR1 was heterogeneously expressed, ranging from highly positive to completely negative cases. Primary samples with an intermediate expression of ROR1 (A); with a high expression of ROR1 (B); and with no ROR1 expression (C).