Whereas the ORM1 locus was polymorphic, the ORM2 locus appeared to be monomorphic. Later, however, Yuasa et al. (1987) demonstrated polymorphism of the ORM2 locus, with 6 alleles in the Japanese population. By studies of orosomucoid cDNA clones isolated from mouse liver, Escallon et al. (1987) presented evidence for the expression of a second structural locus (ORM2) in human plasma as well as for the detection of genetically determined variation at the ORM2 locus in American blacks by the use of a sensitive enzyme-linked immunoblotting technique. After thin-layer polyacrylamide isoelectric focusing (IEF) in gels containing 6 M urea, the isoprotein species coded by the second locus did not segregate with the predominant alleles at the ORM1 locus. Variation in ORM2 was not observed in U.S. whites or in several other populations studied.