Recent studies have shown the importance of autocrine/paracrine production of hGH and hPRL during human breast carcinoma cell proliferation in vitro (Kaulsay et al, 1999; Mertani et al, 2001) and have proposed that local production of PRL in vivo is more important than systemic PRL for the tumour formation (reviewed in Wennbo and Tornell, 2000). The gene discussed is PRL; the disease is breast carcinoma.