The RNA-binding protein XSeb4R regulates maternal Sox3 at the posttranscriptional level during maternal-zygotic transition in Xenopus

► XSeb4R regulates maternal Sox3 at the posttranscriptional level in Xenopus embryo. ► XSeb4R protein binds specifically to the 3′UTR of Sox3 mRNA. ► XSeb4R increases Sox3 mRNA stability and stimulates its translation. ► XSeb4R is required for germ layer boundary delimitation.