Table 1.

Definitions of key terminology.

TERM	DEFINITION
Acetylation	The addition of an acetyl group to lysine amino acid residues
Chromatin	The DNA molecule in combination with the proteins and RNA bound to it
Epigenetic modifications	Stable changes to chromatin structure that do not change the nucleic acid sequence, such as DNA methylation and histone modifications
Epigenetics	The study of epigenetic modifications
Fundus photograph	Picture of the back of the eye, including the retina, macula, fovea, and optic disk
H3K9me3	Histone modification associated with transcriptionally inactive chromatin: three methyl groups attached to the lysine at amino acid position 9 of histone H3
Histone	One of the five proteins that are the primary protein components of chromatin
Heterochromatin	Structurally condensed, transcriptionally inactive regions of chromatin
HP1	Heterochromatin protein 1, protein associated with heterochromatin which maintains the heterochromatic state
Methylation	The addition of a methyl group to cytosine DNA residues; or the addition of one or more methyl groups to histone lysine or arginine amino acid residues
Optical coherence tomography	A method for capturing a cross-sectional image of tissue
Phosphorylation	The addition of a phosphoryl group to lysine, threonine, or tyrosine amino acid residues
Pioneering transcription factors	Transcription factors which bind to repressed, heterochromatic DNA to facilitate activation of a gene
Transcription factor	1) any protein involved in transcription; 2) sequence-specific DNA binding proteins which facilitate the transcriptional activation or repression of target genes
Ubiquitination	The addition of one or more ubiquitin groups to lysine amino acid residues