Two alternatively spliced forms of HAI-2 are found in human tissues. HAI-2A, the full-length molecule and also known as placental bikunin, is a major form expressed in human tissues. HAI-2A consists of two Kunitz domains, and a C-terminal transmembrane domain. Both Kunitz domains can function as inhibitors independent of each other. In addition to HGF activator and trypsin, HAI-2A strongly inhibits plasmin, tissue and plasma kallikreins, and factor XIa.
Three alternatively spliced forms of HAI-2 have been found in mouse tissues. HAI-2A is a minor form expressed in mouse tissues. HAI-2B, with residues 37 through 93 spliced out and thus lack of the first Kunitz domain, is a major form in mouse tissues. HAI-2B is an effective inhibitor of both HGF activator and trypsin.