Abstract: CD93 functional domains for use in treating osteoporosis. A method of alleviating, reducing, suppressing, and/or treating an osteoclast-related bone disease is disclosed. The method comprises administering a therapeutically effective amount of an isolated recombinant protein comprising an amino acid sequence that is at least 80% identical to human Cluster of Differentiation 93 protein domain 1 to a subject in need thereof, the recombinant protein lacking amino acid residues 1 to 21, transmembrane and cytoplasmic domains of the human CD93 (SEQ ID NO: 3) and having a total length of no more than 559 amino acid residues. In one embodiment, the osteoclast-related bone disease is at least one selected from the group consisting of osteoporosis, postmenopausal osteoporosis, osteopenia, bone loss, inflammatory bone loss, and any combination thereof. In another embodiment, the recombinant protein comprises the amino acid sequence of SEQ ID NO: 1 or SEQ ID NO: 2.