G1 phase is the phase of the cell cycle that cells grow and synthesize new proteins and organelles.
G1 phase is the phase between the completion of the mitosis and the beginning of the DNA synthesis in the S phase. In this phase, protein synthesis and production of organelles happen.
Just think about cancer.. it's uncontrolled cell growth, so mutations in genes can cause cancer by accelerating cell division. As the cells contribute to grow they then develop as a tumor.