H3K9me21

H3K9me21 is not a standard histone modification widely recognized in chromatin biology. The term may reflect a typographical error or a study-specific label. In most scientific literature, methylation of lysine 9 on histone H3 is described as H3K9me1, H3K9me2, or H3K9me3.

Methylation at H3K9 exists in three main states with distinct associations and functions. H3K9me1 is less well characterized and is often viewed as a potential precursor to higher methylation states. H3K9me2 is abundant in heterochromatic regions and at some gene loci, generally linked to transcriptional repression. H3K9me3 is a hallmark of constitutive heterochromatin and is strongly associated with long-term gene silencing and genome stability.

The establishment, removal, and interpretation of H3K9 methylation involve coordinated enzymes and reader proteins. Writers include histone methyltransferases such as SUV39H1/H2, SETDB1, and G9a (EHMT2) with its partner EHMT1. Erasers include demethylases from the JMJD2 (KDM4) family. Readers such as HP1 proteins recognize me2 and me3 marks and promote chromatin compaction through interactions with other silencing factors.

Biological significance of H3K9 methylation spans development, differentiation, and genome integrity. These marks help silence repetitive elements, repress unwanted transcription, and participate in processes like X-chromosome inactivation and imprinting. Misregulation is associated with cancers, developmental disorders, and neurological diseases.

If H3K9me21 appears in a source, it should be treated cautiously. Verify whether the authors intended H3K9me2 or H3K9me3, and consult the primary literature for context and nomenclature.