May play a key role in the control of gene expression during oogenesis and early embryogenesis, presumably through the perturbation of chromatin structure. Essential for meiotic maturation of germinal vesicle-stage oocytes. The somatic type linker histone H1c is rapidly replaced by H1oo in a donor nucleus transplanted into an oocyte. The greater mobility of H1oo as compared to H1c may contribute to this rapid replacement and increased instability of the embryonic chromatin structure. The rapid replacement of H1c with H1oo may play an important role in nuclear remodeling. In the germinal vesicle oocyte, localizes to the condensed chromosomes. In the 1-cell embryo found in condensed maternal metaphase chromatin but not in the sperm head. Following second polar body extrusion, detected in the swollen sperm head as well as in the second polar body. Reduced expression in the nucleus in 2-cell embryo is seen as compared to 1-cell embryo. Oocyte-specific. Expressed as early as the germinal vesicle (GV) stage oocyte, and persists into the metaphase II stage oocyte, the oocytic polar bodies, and the 2-cell embryo, and disappears at the 4- to 8-cell embryonic stage. Belongs to the histone H1/H5 family.