FTSJ1 is a 329 amino acid nucleolar protein belonging to the RlmE family and methyltransferase superfamily. Expressed in adult thalamus, hippocampus, amygdala, corpus callosum and caudate nucleus, as well as fetal kidney, lung, liver, brain and lung, FTSJ1 plays a role in rRNA modification and processing. FTSJ1 exists as multiple spliced isoforms which are encoded by a gene located on human chromosome Xp11.23. Notably, defects in the gene encoding FTSJ1 are the cause of mental retardation X-linked type 44 (MRX44) and nonsyndromic X-linked mental retardation (MRX9).