Answer:
The levels of G1 cyclins are too low.
Step-by-step explanation:
During the G1 phase of the cell cycle, each cell makes a fundamental decision: either it continues the cycle and divides or remains in a state of permanent or temporary non-division. Phase G1 is the longest and most variable phase of the cell cycle. If the amount of nutrients is insufficient, or if the amount of cyclins is too low the cell cycle progression will be terminated in G1. In this case, the cell may exit the cycle or move to stage G0, but may not move to stage S.