The invention concerns the human sbg1, g34665, sbg2, g35017 and g35018 genes, polynucleotides, polypeptides biallelic markers, and human chromosome 13q31-q33 biallelic markers. The invention also concerns the association established between schizophrenia and bipolar disorder and the biallelic markers and the sbg1, g34665, sbg2, g35017 and g35018 genes and nucleotide sequences. The invention provides means to identify compounds useful in the treatment of schizophrenia, bipolar disorder and related diseases, means to determine the predisposition of individuals to said disease as well as means for the disease diagnosis and prognosis.