Mice homozygous for a null mutation of activin beta B are viable and fertile.
Mech Dev 1994 Jul;47(1):43-51
We have made a null mutation in the mouse activin beta B gene by deleting the portion of the gene encoding the proteolytic cleavage site and the majority of the coding region for the mature processed protein. Mice homozygous for this mutation complete embryogenesis and are completely viable. Approximately 40% of the homozygous mutant animals are born with open eyes. Aside from the incompletely penetrant eye defects, histopathological analysis has not revealed any other abnormalities in homozygous mutant animals. Breeding tests have shown that both male and female homozygous mutant animals are fertile.
Schrewe, H; Gendron, Maguire M.; Harbison, M L.; and Gridley, T, " Mice homozygous for a null mutation of activin beta B are viable and fertile." (1994). Faculty Research 1990 - 1999. 601.