KusakiDB is a database of orthologous protein sequences in plant species, which merges three critical databases OrthoDB, UniProt, and RefSeq. It has two main functions applied to the orthologous group (OG) among plant genomes. One is the ‘OG Assessment’ that estimates OG(s) by calculating the degree of conservation of protein sequences within a plant family or all plant species. The other is the ‘OG Management’ that estimates the parameters for the assignment of OG(s). Comparing OGs of KusakiDB allows users to find species- and family-specific OG(s) and investigate whether a single OG has multiple functions. It is also expected to assign more accurate gene function according to the functional information of the genes classified into the same OG.