Cell Biology. In the article “Activation of Ca²⁺ signaling in neutrophils by the mast cell-released immunophilin FKBP12” by Holger Bang, Wolfgang Müller, Michael Hans, Kay Brune, and Dieter Swandulla, which appeared in number 8, April 11, 1995, of Proc. Natl. Acad. Sci. USA (92, 3435–3438), the authors request the following correction. “The mast cells used for the experiments published in the above-mentioned publication were not HMC-1 cells but were most likely MC-9 cells from a mouse cell line.”