The anti-MRG-1 antibody used in this article strongly stained mitotic and meiotic germ cells in adult hermaphrodites. However, since the anti-MRG-1 antibody cross-reacts with an unknown protein of 55 kDa, it is not sure whether MRG-1 is a germline-specific protein. Therefore, the levels of MRG-1 was examined by Western blot analysis in three temperature-sensitive mutants with distinct deficiencies in germline development. These mutant animals show the following phenotypes at the restrictive temperature: in
glp-4(
bn2) animals, mitosis of PGCs is very slow because of the reduced function of
glp-4, resulting in only ten to 20 mitotic germ cells and no meiotic cells in the gonad at the young adult stage. In
fem-3(
e2006) animals, the male sex-determination pathway is abolished because of the reduced function of
fem-3, and only oocytes are produced. In
fem-3(
q20) animals, the male sex-determination pathway is constitutively activated because of the gain-of-function of
fem-3, and only sperms are produced. MRG-1 was detected in all of the mutant adult hermaphrodites at the permissive temperature and also in
fem-3(
e2006) mutant hermaphrodites at the restrictive temperature at similar levels as in wild-type hermaphrodites. At the restrictive temperature, MRG-1 could not be detected in
glp-4(
bn2) mutant animals. In
fem-3(
q20) mutant animals at the restrictive temperature, the level of MRG-1 was significantly reduced. These results demonstrate that MRG-1 is expressed predominantly in oocytes.