Administration of unmodified prolactin (U-PRL) and a molecular mimic of phosphorylated prolactin (PP-PRL) during rat pregnancy provides evidence that the U-PRL: PP-PRL ratio is crucial to the normal development of pup tissues. - Yang,L.; Kuo,C.B.; Liu,Y.; Coss,D.; Xu,X.; Chen,C.; Oster-Granite,M.L.; Walker,A.M. (J Endocrinol, 2001)