The issue focused on scientific literature published between 1975 and 1998 that investigated genetic variants linked to schizophrenia and bipolar disorder. Scientists attempted to trace genetic variants linked to schizophrenia by analyzing genetic variants associated with schizophrenia (identified using molecular genetics methods), bipolar disorder (defined using epidemiological methods), or schizophrenia and bipolar disorder (defined using genome-wide association studies). Scientists attempted to trace genetic variants linked to bipolar disorder using genome-wide association studies (GWAS), which compare genetic variants across samples and genetic variants associated with schizophrenia across samples.