Activation of JAK/STAT signalling is a feature of many haematological malignancies and solid tumours. Better understanding of the molecular events contributing to JAK/STAT pathway activation, and a greater range of therapies acting on the pathway, should lead to improved treatment options for patients with malignancies associated with activation of the pathway. This work builds on screens that identified genes and drugs which modulate JAK/STAT signalling in the fruit fly Drosophila melanogaster, to determine whether these effects occur in the conserved human pathway. The gene ANKHD1 (Ankyrin Repeat and KH Domain containing 1) has been identified as a positive regulator of JAK/STAT signalling in Drosophila, but there has been little investigation of ANKHD1 in human tissues. I show that ANKHD1 protein is expressed in normal blood cells and the malignant clone of cells in acute leukaemias. I also show that ANKHD1 protein is expressed in the skin cancer malignant melanoma, where it is found in the nucleus and cytoplasm in a range of histological sub-types of melanoma. Methotrexate was identified as a selective suppressor of JAK/STAT signalling in a screen in Drosophila cells. No interaction between methotrexate and JAK/STAT signalling has previously been described. I demonstrate that methotrexate suppresses JAK/STAT signalling in human cell lines at drug concentrations equivalent to those measured in patients, suggesting that suppression of JAK/STAT signalling may contribute to the mechanism-of-action of methotrexate in inflammatory conditions. Furthermore, the effect occurs when the pathway is activated by the JAK2V617F mutation found in patients with myeloproliferative neoplasms (MPNs). The potential of methotrexate as a treatment for patients with MPNs is investigated in primary cells obtained from patients with myelofibrosis.