AbstractThe process of allorecognition consists of an ability to discriminate self from non‐self. This discrimination is used either to identify non‐self cells and reject them (“non‐self histocompatibility”) or to identify self cells and reject them (as in the avoidance of self‐fertilization by hermaphrodites (“self incompatibility”). The molecular basis governing these two distinct systems has been studied recently in hermaphroditic ascidian urochordates. Harada et al.1 postulated two highly polymorphic self‐incompatibility loci, Themis (A and B), that are transcribed from both strands, forward to yield sperm (s‐) trans‐membrane antigen, and reverse to yield the egg vitelline coat (v‐) receptor. De Tomaso et al.2 characterized a candidate histocompatibility locus, encoding a highly variable immunoglobulin. Nyholm et al.3 isolated its candidate allorecognition receptor, fester. Only a minute similarity was found in the structure of the genes involved. It appears that ascidian harbor two very separate types of labeling and recognition genetic systems: one for self and the other for non‐self. BioEssays 30:1048–1051, 2008. © 2008 Wiley Periodicals, Inc.