The myelodysplastic/myeloproliferative neoplasms (MDS/MPN) are clonal myeloid neoplasms characterized at the time of their initial presentation by the simultaneous presence of myelodysplastic and myeloproliferative features, which prevent them from being classified as either myelodysplastic syndrome (MDS) or myeloproliferative neoplasm (MPN). The incidence of MDS/MPN is estimated at 0.1 to 3/100,000 individuals. They are characterized by hypercellular bone marrow (BM) morphology due to proliferation in one or more of the myeloid lineages. Cytopaenias and dysplastic changes of any cell line may be seen in conjunction with elevated white blood cell (WBC) counts, thrombocytosis and organomegaly, features more commonly associated with MPN. Hepatosplenomegaly is frequently seen. The most common entities within the MDS/MPN group include chronic myelomonocytic leukaemia (CMML), atypical chronic myeloid leukaemia BCR-ABL1 negative (aCML) and juvenile myelomonocytic leukaemia (JMML), which is seen exclusively in paediatric patients. A less well-defined group of MDS/MPN-like diseases includes MDS/MPN unclassifiable (MDS/MPN-U) and a recently recognized entity of MDS/MPN with ring sideroblasts and thrombocytosis (MDS/MPN-RS-T), previously known as refractory anaemia with ring sideroblasts and thrombocytosis (RARS-T). It was considered a provisional entity within the group of MDS/MPN-U in the 2008 edition of the WHO, but has now been promoted to a true entry in the updated 2016 WHO edition. Since the publication of the last WHO Classification in 2008, multiple studies have examined the molecular pathogenetic features of the MDS/MPN entities (see Table 11.1). Many of these results have been incorporated into the updated 2016 WHO classification.

Myelodysplastic syndromes (MDS) comprise a heterogeneous group of myeloid neoplasms defined by peripheral cytopaenia, morphologic dysplasia in one or more haematopoietic lineages, and genetic instability with risk of transformation to acute myeloid leukaemia (AML). A diagnosis of MDS requires the presence of cytopaenia, plus one or more of the following diagnostic features: morphologic evidence of dysplasia in >10% of cells involving at least one haematopoietic lineage, increased blasts, and/or an MDS-defining cytogenetic abnormality. Modern diagnosis of MDS was standardized by the French–American–British (FAB) cooperative group in 1982, which included the first standard descriptions of myelodysplasia [1], and was further refined in the World Health Organization (WHO) classification of 2001, which more specifically quantified dysplasia (Table 8.1) [2].