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 (By similarity). In the 1-, 2- and 4-cell embryo, it is uniformly spread within the cytoplasm, while it is concentrated onto the chromatin in the nucleus. In the 8- to 16-cell embryo, its presence diminishes in the cytoplasm, although it is still strongly expressed in the nucleus. Oocyte (at protein level). Found in the germinal vesicle (GV) oocyte and diminishes constantly throughout embryonic development. The levels compared to the initial amount found in the GV oocyte decreases to 41%, 28% and 7% in the 2, 4, and 8-cell embryos. In the 16-cell embryo and blastocyst, respectively, levels are 200 and 2000 times lower than in the GV oocyte. Belongs to the histone H1/H5 family.