The thawing procedure is critical for NK cell viability and expansion. (A) Workflow of the thawing of frozen NK cells as described in Materials and methods. (B) NK cells treated with or without DNase I at 48 h after thawing. (C) Microscopic images of NK cells show the change in cell morphology and formation of large clusters as the expansion progressed. Scale bars indicate 200 µm. Scale bars in the zoomed panels indicate 20 µm.