Final answer:
Cyclins are the family of proteins that regulate the timing of the cell cycle in eukaryotic cells by binding to and activating cyclin-dependent kinases (Cdks).
Step-by-step explanation:
The family of proteins that regulate the timing of the cell cycle in eukaryotic cells is called Cyclins. Cyclins are responsible for the progress of the cell through the various checkpoints in the cell cycle. They bind to and activate a group of enzymes called cyclin-dependent kinases (Cdks), which phosphorylate other proteins and control the advancement of the cell cycle.