Clonal and malignant T_HS cells are present in the blood of patients with L-CTCL. (A) T_HS cells were present in the blood of patients with CTCL with known blood involvement but absent in patients without evidence of peripheral blood disease. Representative patients are shown; a T_HS cell population has been confirmed in 16 patients with L-CTCL. (B) Blood T_HS cells were obscured by monocytes in some patients. Staining for CD3 in addition to CD4 was required to show these cells. (C) T_HS cells were uniformly clonal and malignant in patients with L-CTCL and identifiable malignant T-cell clones. Shown are 8 patients with L-CTCL with identifiable T-cell clones. T_HS cell populations (red) were present in all patients. Selective gating on T_HS cells showed that most were clonal malignant T cells. SSC-A indicates side-scatter area; FSC-H, forward-scatter height.