RSPO1 and disorder of sexual differentiation: Similarly in humans, rare cases of XY DSD caused by a genomic duplication involve the locus encompassing RSPO1 and WNT4. While FOXL2 expression in the ovary is highly conserved throughout vertebrate species, its functions in either primary sex determination, ovarian maintenance or folliculogenesis has evolved within vertebrate clades, and even within species from the same clade.