Peroxisomes and histones were targeted in the pig kidney cells featured in the digital video sequences presented in this section with mCherry fused to a peroxisomal targeting signal and mEmerald fused to H2B, respectively. Peroxisomes are organelles that require significant amounts of oxygen in order to function properly, a characteristic that would have been especially important millions of years ago before cells contained mitochondria and the atmosphere of Earth was first becoming oxygen-rich. Since it was poisonous to most forms of life, levels of oxygen within cells needed to be minimized in order for early cells to survive. In the absence of mitochondria, the task of detoxifying cells of oxygen would have been primarily the responsibility of peroxisomes. Later, mitochondria would also utilize oxygen to carry out a variety of reactions, but with the added benefit of generating energy in the form of ATP.