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In the ABC model, MADs-box proteins play an important role in floral development. As one of the MADs-box proteins, PI behaves as B function organ-identity genes acting in the second and third whorls of the flower to control petal and stamen identity. To find genes involved in the floral transition of pineapple guava, we cloned the full length cDNA encoding a MADs-box protein in Coolidge pineapple guava, using RT-PCR and RACE (Rapid Amplification of cDNA Ends) strategy. The MADs-box gene was denominated FsMADs-box and belonged to a PI homolog. It was specifically expressed in flower buds, petal, and stamen, but not in leaves, sepal, carpel, and fruits in different developmental stages. The full length FsMADs-box cDNA contained a complete open reading frame (ORF) of 624 bp encoding 208 amino acids. The FsMADs-box gene showed a conserved MADS motif, a less conserved K-box motif, and MIKC structure that was typical for MADs-box proteins from high eudicots. The molecular weight (Mw) and isoelectric point (pI) of FsMADs-box were predicted to be 24.3 kDa and 8.83, respectively. In comparison with other PI homologs at the deduced amino acid level, the FsMADs-box showed the highest similarity (92%) with MqMADS-box of M. quinquenervia, and the lowest (65%) with HmPI of H. macrophylla. The present study provided the basis for further dissection of the FsMADs-box gene function in governing the transition from the juvenile to the adult stage.