Complex sentence: Histone H1 is one of the 5 main histone protein families which are components of chromatin in eukaryotic cells, and while highly conserved is nevertheless the most variable histone in sequence across species.
Split into simpler sentences: Histone H1 is one of the five main histone protein families which are components of chromatin in eukaryotic cells. Though highly conserved, it is nevertheless the most variable histone in sequence across species.