cDNA clones encoding a novel activin β subunit have been isolated from mouse liver cDNA library. An amino acid homology comparison among members of the TGF-β superfamily indicates that the open reading frame codes for a new activin/inhibin β subunit. The predicted mature region of the subunit shows >60% identity with activin β C and β D and ≥45% identity with activin β A and β B , but only 20-40% amino acid sequence identity with other TGF-β superfamily members. Therefore, the novel subunit has been designated activin β E . Alignment of the mature growth factor region of the five activin β subunits revealed that activin β A and β B share 63% amino acid identity, while activin β C , β D , and β E share 62% identity. Therefore, activin β A and β B are most related to one another, while activin β C , β D , and β E may represent a subset of related sequences.