As further evidence of the importance of the pituitary clock, circadian expression of pituitary POMC and PRL mRNA was observed in cultured murine pituitary tumor cell lines (AtT20, GH3 and GH4C1), and their expression correlated to the circadian proteins (PER-2 for POMC, and CLOCK for PRL) [89,90]. This evidence concerns the gene PRL and pituitary tumor.