The response of renal epithelial cells to injury can include hyperplasia (increase in cell number), apoptosis (cell death), antiproliferation (growth arrest), or hypertrophy (cells physically enlarge). Examining cell size and the protein:DNA ratio can differentiate between the growth response patterns, but it is proposed that the degree of activation of cyclin D kinase in the late G1 phase of the cell cycle differentiates between hyperplasia and hypertrophy.