E1A Binding Protein P300 (EP300) is a transcriptional co - activator. It has histone acetyltransferase activity, which can modify chromatin structure by adding acetyl groups to histones. This modification affects gene expression, as it generally relaxes chromatin, allowing transcription factors better access to DNA. EP300 interacts with numerous transcription factors and is involved in various cellular processes like cell growth, differentiation, and apoptosis. Mutations in EP300 can lead to certain genetic disorders and are associated with an increased risk of some cancers.According to the literatures, EP300 mediates cAMP-gene regulation by binding specifically to phosphorylated CREBBP.Thus a functional ELISA assay was conducted to detect the interaction of recombinant human EP300 and recombinant human CREBBP. Briefly, EP300 was diluted serially in PBS with 0.01% BSA (pH 7.4). Duplicate samples of 100 μl were then transferred to CREBBP-coated microtiter wells and incubated for 1h at 37℃. Wells were washed with PBST and incubated for 1h with anti-EP300 pAb, then aspirated and washed 3 times. After incubation with HRP labelled secondary antibody for 1h at 37℃, wells were aspirated and washed 5 times. With the addition of substrate solution, wells were incubated 15-25 minutes at 37℃. Finally, add 50 µL stop solution to the wells and read at 450/630nm immediately. The binding activity of recombinant human EP300 and recombinant human CREBBP was shown in Figure 1, the EC50 for this effect is 0.023ug/mL.