In neurons, shows punctate expression throughout the cell body, nucleus and dendrites. Not detected in axons. Widely expressed in brain where it localizes to the olfactory bulb, forebrain, thalamus, hippocampus, cerebellum, cortex and hypothalamus (at protein level) (PubMed:12691844). Expression seems to be particularly strong in areas of high synaptic density such as the glomerular layer of the olfactory bulb, and mossy fiber terminal fields of the hippocampus (at protein level) (PubMed:12691844). Expressed in retina, with strongest expression in the external and internal plexiform layers (at protein level) (PubMed:12691844). Strongly expressed in brain and testis (PubMed:7633444, PubMed:1302022). Also detected at lower levels in heart, kidney, liver, lung, ovary, uterus, bladder and skeletal muscle (PubMed:7633444, PubMed:1302022). In testis, expression seems to be restricted to secondary spermatocytes (PubMed:1302022). In brain, shows increasing expression levels from postnatal day 7 onwards reaching peak levels by postnatal day 21 (at protein level) (PubMed:12691844). Expression is seen very early in embryogenesis, 9.5 dpc in all parts of the embryo (PubMed:7633444). Mid-gestation embryos (14.5 dpc) show high expression in all neural tissues such as telencephalon, retina and spinal cord (PubMed:7633444). In the 16.5 day old embryo high expression is seen in the neural tissues, like telencephalon and mesencephalon and in the non-neural tissues, such as muscle tissues in the tongue and around the ribs (PubMed:7633444). It is uncertain whether Met-1 or Met-16 is the initiator.