Which histone is not included in the nucleosome octamer?

The histone that is not part of the nucleosome octamer is H1. The nucleosome is the fundamental unit of chromatin and consists of a core octamer composed of two copies each of histones H2A, H2B, H3, and H4. This structure serves to package DNA in a compact form, facilitating regulation of transcription, replication, and DNA repair.

Histone H1, in contrast, is known as the linker histone, and its primary role is to bind to the outside of the nucleosome. It stabilizes the nucleosome structure and helps to compact the chromatin further by linking adjacent nucleosomes together, but it does not contribute to the core octamer that is essential for the formation of the nucleosome itself. Therefore, H1 is not included within the octamer structure of the nucleosome, making it distinct from the other histones that form the core.