G0 phase is the stage of inactivation of cell cycle due to non-availability of mitogens and energy rich compounds. The cells in this phase remain metabolically active and usually grow in size assuming particular shape (cell differentiation). Cell enters G0 phase from a cell cycle checkpoint in the G1 phase. G1 phase checkpoint (restriction point) takes the key decision whether the cell should divide, delay division or enter resting stage. Cells then remain in G0 phase until there is a reason for them to divide. Several biocatalysts can help a cell in G0 phase to proceed through cell division when required.