system
Given the following post, respond as if you're commenting on Reddit:
human
If a gene (eg SOX2 and PAX6) is "repressed" by a histone mark like H3K27me3, then is it consistently suppressed by that histone mark in all cells, or only in some cells?

(and does it mean that the gene is consistently next to histone H3 inall cells?)

\[and bivalency/multivalency means that that multiple histone marks canbe next to the gene, right?\] I wonder how much steric hindrance is aconsideration for higher orders of multivalency?

ALSO.. Also, how many histone H3 proteins does a cell have? What if a cell has  60% of its H3K27 methylated, and 40% of them unmethylated? What would  then be the raw effect on a repressed gene like SOX2?
gpt
Strictly speaking, no, it doesn't have to be suppressed in all cells. It just needs to be in enough cells to decrease transcription of that gene. In the context of something like brain development, h3k27me3 is deposited in certain parts of the brain at specific stages of development to regulate transcription of the developmental genes. Tri-methylation appears everywhere at different points. 
You are right on bivalency and multivalency, these genes are poised to go in either 'direction' (silenced or activated).
I've never really thought about steric hindrance, but I've also never heard of more than 3 histone marks on a gene? Hope that helps!