Table 1. Definitions for distinct modification types and quantitative MS measurements used in the study.

Modification type	Meaning	Examples
Single PTM	Specified aa residue within H3 N-terminal tail is modified by a given PTM; other aa residues are unmodified	H3K9me2: H3K9 is dimethylated; other H3 N-terminal tail aa residues are unmodified
Individual PTM	Specified aa residue within H3 N-terminal tail is modified by a given PTM; other aa residues can either be modified or unmodified	H3K9me2: H3K9 is dimethylated; other H3 N-terminal tail aa residues can either be modified or unmodified
Combinatorial PTM	Specified aa residues within H3 N-terminal tail are modified by a given PTM(s); other aa residues are unmodified	H3K9me2K27me3: H3K9 and H3K27 are di- and trimethylated, respectively; other H3 N-terminal tail aa residues are unmodified
Binary PTM	Two specified aa residues within H3 N-terminal tail are modified by given a PTM(s); other aa residues can either be modified or unmodified	H3K9me2K27me3: H3K9 and H3K27 are di- and trimethylated, respectively; other H3 N-terminal tail aa residues can either be modified or unmodified
Quantitative MS measurement	Meaning	Examples
Relative abundance (RA) of PTM on total H3	Proportion (%) of all H3 N-terminal tails carrying a given PTM	It is important to distinguish the difference between the RAs of single and individual PTMs:
RA of PTM on H3.3 (H3.1/2)	Proportion (%) of H3.3 (H3.1/2) N-terminal tails carrying a given PTM	RA of a single PTM H3K9me2 is 1% means that 1% of all H3 N-terminal tails is modified by K9me2 with NO other modifications present on them.
		RA of an individual PTM H3K9me2 is 1% means that 1% of all H3 N-terminal tails is modified by K9me2 alone or in combination with any other H3 PTM(s).