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Background: Somatostatin (SST) and its receptors (SSTR1-5) may have a role in prostate cancer by influencing the IGFI hormone axis or through direct effects on prostate epithelia. We have investigated if genetic variation in the SST and SSTR1-5 genes influences prostate cancer risk and/or circulating IGFI and IGFBP3 hormone levels. Materials and Methods: We analyzed 28 haplotype tagging single nucleotide polymorphisms in the SST and SSTR1-5 genes in a case-control/genetic association study to investigate the association between genetic variation and prostate cancer risk. The study included 2863 cases and 1737 controls from the Cancer Prostate in Sweden (CAPS) study. To investigate the genetic influence on circulating hormone levels, plasma concentrations of IGFI and IGFBP3 were analyzed in 874 controls of the CAPS study and 550 male subjects from the Northern Sweden Health and Disease Cohort (NSHDC). Results: No clear association between prostate cancer risk and genetic variation of the SST and SSTR1-5 genes was identified. The SSTR5 missense single nucleotide polymorphism rs4988483 was associated with circulating IGFI (P = 0.002) and IGFBP3 (P = 0.0003) hormone levels in CAPS controls, with a per allele decrease of similar to 11%. This decrease was replicated in NSHDC for circulating IGFBP3 (P = 0.01) but not for IGFI (P = 0.09). Combining CAPS and NSHDC subjects indicated evidence of association between rs4988483 and both IGFBP3 (P = 2 x 10(-5)) and IGFI (P = 0.0004) hormone levels. Conclusions: Our results suggest that genetic variation in the SSTR5 gene and, particularly, the rs4988483 single nucleotide polymorphism influence circulating IGFI and IGFBP3 hormone levels with no measurable effect on prostate cancer risk. (Cancer Epidemiol Biomarkers Prev 2009;18(5):164.4-50)