EPO and major depressive disorder: Another pathway implicated in depression with effects on neuroplasticity and neurogenesis is under the regulation of the growth factor erythropoietin, both independent and dependent on its role in haematopoiesis.46, 47 Our results provide some evidence that erythropoietic pathways are altered in MDD individuals with the three most significantly upregulated probe sets representing two genes, HBA and HBB (haemoglobin alpha and beta).