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Primary breast cancer biopsy tissue is not always available to assess the degree of immune-cell infiltration into HER2⁺ breast cancer tumors. Thus, one goal for cancer diagnostics is to find reliable measures based on circulating immune cells. Muntasell and Servitja et al. measure the absolute number of circulating CD57⁺ NK cells before treatment with HER2-specific antibodies and find that the higher the number of NK cells before treatment, the worse the patient's response to treatment. High amounts of CD57⁺NK cells in the blood inversely correlate with their density in tumors, suggestive of a tumor-homing or proliferative deficiency. Indeed, the authors find that CD57⁺ NK cells in the peripheral blood have both reduced tumor-homing chemokine receptor expression and low proliferation upon activation. The amount of these circulating cells could be used as a diagnostic marker of response to HER2 antibodies. Read more in this issue on page 1280. Original image from Fig. 3B. Artwork by Lewis Long.