Fluorescent indicators for simultaneous reporting of all four cell cycle phases

The far-red fluorescent protein mMaroon1 and a reporter based on stem-loop binding protein enables the generation of Fucci4, a 4-color cell cycle reporter system that can be used to distinguish all phases of the cell cycle. Also online, a paper by Laviv et al . uses mMaroon1 as a FRET acceptor for the newly developed CyRFP1. A robust method for simultaneous visualization of all four cell cycle phases in living cells is highly desirable. We developed an intensiometric reporter of the transition from S to G2 phase and engineered a far-red fluorescent protein, mMaroon1, to visualize chromatin condensation in mitosis. We combined these new reporters with the previously described Fucci system to create Fucci4, a set of four orthogonal fluorescent indicators that together resolve all cell cycle phases.