This is a family of small, approximately 100 amino acid, proteins found from yeasts to humans. The majority of endogenous reactive oxygen species (ROS) in cells are produced by the mitochondrial respiratory chain. An increase or imbalance in ROS alters the intracellular redox homeostasis, triggers DNA damage, and may contribute to cancer development and progression. Members of this family are mitochondrial reactive oxygen species modulator 1 (Romo1) proteins that are responsible for increasing the level of ROS in cells. Increased Romo1 expression can have a number of other effects including: inducing premature senescence of cultured human fibroblasts and increased resistance to 5-fluorouracil.