However, clinical observations of the relationship between PTH and prolactin levels are conflicting, and processes in vivo such as e.g. tumour induced hyperprolactinemia, pregnancy, the menstrual cycle and polycystic ovary syndrome have proved too complex to establish a consensus model of causality [6], [7], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]. This evidence concerns the gene PRL and hyperprolactinemia.