MEKK1^–/–-derived neutrophils display altered cell migratory behavior, with periods of hyperextension related to impaired release of their trailing edge. The migratory behavior of wild-type and MEKK1^–/– ES/OP9-derived neutrophils was assessed by videotaping cell migration using a Zigmond chamber. As seen in panel A, neutrophils are migrating toward the gradient, which is on the left with a normal morphology. In panel B, the majority of MEKK1^–/–-derived neutrophils are also migrating toward the left, frequently exhibiting impaired tail release and hyperextension (H). Original magnification × 280.