Binds to the GM-CSF promoter. Seems to act as a repressor (By similarity). Binds also to full-length mRNA and to short RNA sequences containing the consensus site 5'-UCCAUCA-3'. May have a role in translation repression. Found in a mRNP complex with YBX2 (PubMed:10772793). Interacts with RRP1B (By similarity). Expressed in oocytes, spermatocytes and spermatids (at protein level). Expressed in skeletal muscle, kidney, brain, spleen, liver, heart and spermatids. Isoform 2 is preferentially expressed in somatic tissues (PubMed:10956549). Transgenic mice overexpressing Ybx3 exhibit disruption of the normal completion of spermatogenesis, dominant sterility and abnormal translation activation of repressed mRNA.