Fold change (FC) is a measure describing the degree of quantity change between final and original value. For instance, for a data set with an original value of 20 and a final value of 80, the corresponding fold change is 3, or in common terms, a three-fold increase. Fold change is computed simply as the ratio of the changes between final value and the original value over the initial value. Thus, if the original value is X and final value is Y, the fold change is (Y - X)/X or equivalently Y/X - 1.