Growth Factors Bring Cells Out of
G0

G0 is  a resting state really outside of the cell cycle often termed quiescence.

Some cells enter G0 and never re-enter the cycle such as mature neurons.

Some cells enter and leave G0 as the body requires such as stem cells of blood and epithelial tissues.

Some cells stay in G0 for many years and only re-enter the cell cycle in response to very specific signals such as memory B- and T-lymphoid cells that  respond to specific antigens. 

Restriction point (R-point)

Restriction point is a point of no return, in the sense  that the cells are committed to S-phase entrance and growth factor stimulation is no longer required  to maintain the cells in cycle.

So,  growth factor stimulation is essential to get to the R-point but the growth factors can be removed after the R-point and the cells will move through S-phase, G2 and M-phase. 

There are checkpoints, such as DNA damage, that can keep cells in G1

even after they pass the R-point, but under normal conditions the R-point

is a point of no return and the cells will enter S-phase. 

Early and Delayed Transciption

Both Early and Delayed transcription require growth factor stimulation of cells.

*Early gene transcription does not require new protein  

  synthesis. 

Everything required to induce early gene expression is present; it is a matter of activation by signaling cascades in the cells.

*Delayed gene expression requires both growth factor  

  stimulation and new protein synthesis. 

Delayed gene expression requires synthesis of the proteins from some of the early genes.

Myc

-Early Gene Critical in Cell Cycle Progression