S100A10 and major depressive disorder: p11, or S100A10, a member of the S100 family of proteins with 2 EF-hand calcium-binding motifs, is recently reported to play a crucial role in the onset of depression by interacting with the 5-hydroxytryptamine receptor 1b (5-HTR1b) or 5-hydroxytryptamine receptor 4 (5-HTR4)17, 18.