Figure 1.

Schematic of (a) mammalian and (b) budding yeast G1–S control circuits indicates a common feedback-driven regulatory architecture. Shapes correspond to the type of protein (e.g. upward triangles denote cyclin-dependent kinases). Colour implies that the G1–S regulator has high sequence similarity (indicating homology, table 1) to the same regulator in another kingdom (animal, fungi, plant). The G1–S circuit in mammals is colourful (compared with budding yeast) because there are many identifiable sequence homologues between plants and animals.