The presence of macroprolactin or ‘big–big’ prolactin, a heterogenous complex of prolactin and immunoglobulin A (150–170 kDa) that cross-reacts in the total prolactin assay, can lead to over-investigation of hyperprolactinaemia; this benign condition is the apparent cause of hyperprolactinaemia in about 20% of cases [52,53]. This evidence concerns the gene PRL and hyperprolactinemia.