Histone H1 zero: a major player in cell differentiation?

Histone H1 zero was initially described as a member of the lysine-rich histone class, typically present in nondividing mammalian cells. Since then it has been found in almost every animal or plant species studied. The protein accumulates in terminally differentiated cells that have stopped dividing. It has also been implicated in changes in chromatin structure and function accompanying malignant transformation. Despite its involvement in these fundamental cellular processes, its precise role remains elusive, as do the molecular mechanisms via which it acts. This review is an attempt to summarize and critically discuss the huge relevant literature, trying to highlight the problems that still await answers.