Glycoside hydrolase family 18 (GH18) is a phylogenetically conserved group of proteins present in eukaryotes, prokaryotes and viruses. The GH18 family is characterized by a Glyco_18 domain adopting an (a ⁄ b) 8 triose phosphate isomerase-barrel structure that consists of a specific arrangement of eight parallel b-strands, forming the barrel core, surrounded by eight a-helices [1]. This family classification, based only on similarities in amino acid sequences, groups together chitinases and proteins devoid of catalytic activity due to the substitution of a critical amino acid in the catalytic centre. This latter singular class of proteins, called chitinase-like proteins (CLPs), has been identified only recently in plants [2], mammals [3], insects [4] and molluscs [5]. CLPs have been implicated in many biological processes, such as control of nodulation [2] and growth ⁄ differentiation balance during development in plants [6]. Insect CLPs such as imaginal disc growth factors represent the first proliferating polypeptides reported from invertebrates [7]. These mitogenic growth factors cooperate with insulin to stimulate proliferation, polarization and mobility of imaginal disc cells in vitro. Imaginal disc growth factors may also constitute morphogenetic factors controlling embryonic and larval development, and could stimulate the cell growth required for wound healing [8,9]. In mammals, CLPs such as YM1 ⁄ 2 and YKL-40 (40 kDa mammalian protein with the N-terminus YKL) [also known as human cartilage glycoprotein-39 (HC-gp39) in humans] are considered to be cytokines [10,11] Chitinase-like proteins have been identified in insects and mammals as nonenzymatic members of the glycoside hydrolase family 18. Recently, the first molluscan chitinase-like protein, named Crassostrea gigas (Cg)-Clp1, was shown to control the proliferation and synthesis of extracellular matrix components of mammalian chondrocytes. However, the precise physiological roles of Cg-Clp1 in oysters remain unknown. Here, we report the cloning and the characterization of a new chitinase-like protein (Cg-Clp2) from the oyster Crassostrea gigas. Gene expression profiles monitored by quantitative RT-PCR in adult tissues and through development support its involvement in tissue growth and remodelling. Both Cg-Clp1-and CgClp2-encoding genes were transcriptionally stimulated in haemocytes in response to bacterial lipopolysaccharide challenge, strongly suggesting that these two close paralogous genes play a role in oyster immunity.Abbreviations Cg-Clp1 ⁄ 2, Crassostrea gigas chitinase-like protein 1 ⁄ 2; CLP, chitinase-like protein; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GH, glycoside hydrolase; HC-gp39, human cartilage glycoprotein-39 (also called YKL-40); LPS, lipopolysaccharide; YKL-40, 40 kDa mammalian protein with the N-terminus YKL.