Refractory anaemia with ring sideroblasts and marked thrombocytosis (RARS-T) is a provisional entity in the World Health Organization (WHO) classification. It displays features characteristic of both myelodysplastic syndrome and myeloproliferative neoplasia plus ring sideroblasts ≥15% and marked thrombocytosis. Most patients with RARS-T show a normal karyotype. We report a 76-year-old woman diagnosed with RARS-T (76% of ring sideroblasts) with JAK2 (V617F) mutation and a load of 30–40%. Classical and molecular cytogenetic (FISH) studies of a bone marrow sample revealed the presence of isodicentric X chromosome [(idic(X)(q13)]. Moreover, HUMARA assay showed the idic(X)(q13) as the active X chromosome. This finding was correlated with the cytochemical finding of ring sideroblasts. To our knowledge, this is the first reported case of an active isodicentric X in a woman with RARS-T. 1. Introduction Refractory anaemia with ring sideroblasts and marked thrombocytosis (RARS-T) is a rare entity which displays features characteristic of both myelodysplastic syndrome and myeloproliferative neoplasia plus ring sideroblasts ≥15% and marked thrombocytosis. Most patients with RARS-T show a normal karyotype in bone marrow sample [1]. Seven years after its incorporation into the 2001 World Health Organization (WHO) classification [2], RARS-T still remains a provisional entity under the category of myelodysplastic/myeloproliferative neoplasm, unclassifiable [3]. An isodicentric X chromosome with breakpoints in Xq13 (idic(X)(q13)) is a rare cytogenetic abnormality with an extra dose of Xpter-q13 and loss of the Xq13-qter region. Currently, controversy exists regarding whether idic(X)(q13) is active or inactive and whether or not there is any correlation with the cytochemical finding of ring sideroblasts [4]. 2. Case Report We report a 76-year-old female presenting with RARS-T and idic(X)(q13) in bone marrow cells. She presented with anaemia (mean corpuscular volume, 103？fl; haemoglobin, 9.2？g/dL), leukocytes of , and a thrombocytosis of . The patient was diagnosed with RARS-T (76% of ring sideroblasts) with JAK2 (V617F) mutation and a load of 30%–40%. A cytogenetic analysis of the bone marrow revealed two clones, one with an isodicentric X and another with a normal chromosomal complement: 46,X,idic(X)(q13)[4]/46,XX[16] (Figure 1(a)). HUMARA assay showed the idic(X)(q13) as the active X chromosome. In situ fluorescent hybridisation with a DXZ1 centromere probe (Vysis Downers Grove, IL, USA) (Figure 1(b)) and a whole chromosome painting (WCP) X chromosome probe (Vysis
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