JNeurosci is publishing an Expression of Concern for the article, “Transient Hypoxia Stimulates Mitochondrial Biogenesis in Brain Subcortex by a Neuronal Nitric Oxide Synthase-Dependent Mechanism,” by Diana R. Gutsaeva, Martha Sue Carraway, Hagir B. Suliman, Ivan T. Demchenko, Hiroshi Shitara, Hiromichi Yonekawa, and Claude A. Piantadosi, which appeared on pages 2015–2024 in the February 27, 2008 issue. The editors have been made aware of questions about the reliability of the data presented in Figures 1A, 4A, 5B, and 9C. Per journal policy, the authors were contacted to provide an explanation and/or original data for the editors to review and determine if corrective actions were required. The authors have been unresponsive to requests for information, both from the journal office and from the corresponding author's institution. Until more information can be provided and independently evaluated, the Editorial Board cautions readers that the results reported within the article may not be reliable.