 Therefore, Sam68 plays a critical role in regulating synapse number via control of postsynaptic β-actin mRNA metabolism.Neuron-specific CELF family RNA-binding protein UNC-75 specifically binds to the UUGUUGUGUUGU mRNA stretch via its three RNA recognition motifs for the exon 7a selection in "C. elegans"' neuronal cells.As exon 7a is skipped due to its weak splice sites in non-neuronal cells, UNC-75 was found to specifically activate splicing between exon 7a and exon 8 only in the neuronal cells.