ESP is a family of largely rodent exocrine gland-secreting peptides that are produced by the male extraorbital lacrimal gland to be secreted into the tear fluid. Other mice including females detect these peptides through receptors in the vomeronasal organ, and the receptors report information on mouse-strain, sex and species [1]. The peptides are short, all carrying an N-terminal signal-peptide to indicate they are for secretion which accounts for much of the common conservation.

1: Sex- and strain-specific expression and vomeronasal activity of mouse ESP family peptides. Kimoto H, Sato K, Nodari F, Haga S, Holy TE, Touhara K; Curr Biol. 2007;17:1879-1884. PMID:17935991