H1oo: a pre-embryonic H1 linker histone in search of a function. The mouse oocyte-specific linker histone H1oo (1) constitutes a novel mammalian homologue of the oocyte-specific linker histone B4 of the frog and of the cs-H1 linker histone of the sea urchin; (2) is expressed as early as the germinal vesicle (PI) stage oocyte, persisting into the MII stage oocyte, the oocytic polar bodies, and the 2-cell embryo, extinction becoming apparent at the 4-8 cell embryonic stage; and (3) may play a key role in the control of gene expression during oogenesis and early embryogenesis, presumably through the perturbation of chromatin structure.