A 323-bp DNA fragment (U15557) was isolated, cloned, and sequenced after polymerase chain reaction (PCR) amplification from Monodelphis domestica genomic DNA. A HindIII restriction fragment length polymorphism was identified in this species using the U15557 PCR, fragment as a hybridization probe. DNA samples exhibited either a 6.4 kb band, a 7.2 kb band, or both bands simultaneously. Behaviour of these two variants in family studies was consistent with codominant autosomal inheritance. Linkage between this marker and the loci encoding protease inhibitor (PI) and adenylate kinase 1 (AK1) was found in M. domestica.