The cells need continuously oxygen, so for them it is essential to adapt as soon as possible to the least change that occurs in the oxygen levels. They do this by substituting the protein COX4 of mitochondria. In the Johns Hopkins Institute they have seen that it has two different forms and that in normal oxygen conditions the concentration of form COX 4-1 is greater than that of COX 4-2. However, in situations where oxygen is scarce, they have seen that this second form is the one that activates the cell. Apparently, the COX 4-2 is capable of making a more efficient use of energy.