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Cyclin A₂ is a prognostic indicator in early‐stage cancer. However, since most of peptide‐protein bindings do not produce an easily measurable output signal, this severely hinders homogeneous detection of protein using peptide as detection probe. Cyclin A₂ can be detected as low as 0.6 μM using a well‐known p21^(WAF‐1) consensus senquence for a specific cyclin A₂ binding motif, which is incorporated into a Tb³⁺ chelating macrocycle by chemical modification at the N‐terminus of CMB. Herein, a simple, ultra‐sensitive, and selective signal‐on fluorescence assay is developed for detection of a prognostic indicator in early‐stage cancer, cyclin A₂. Graphene oxide (GO) is even superior to SWNTs for cyclin A₂ detection. The direct detection limit using graphene oxide is 0.5 nM, 10‐fold better than using SWNTs, and 1200‐fold better than the latest reported value of 0.6 μM using the Tb³⁺ chelating macrocycle …