Background : Myelodysplastic syndrome (MDS) is a group of disorders involving he-mopoietic dysfunction leading to leukemia. Although recently progress has been made in identifying underlying genetic mutations, many questions still remain. Animal models of MDS have been produced by introduction of specific mutations. However, there is no spontaneous mouse model of MDS, and an animal model to simulate natu-ral MDS pathogenesis is urgently needed.Methods : In characterizing the genetically diverse mouse strains of the Collaborative Cross (CC) we observed that one, designated JUN, had abnormal hematological traits. This strain was thus further analyzed for phenotypic and pathological iden-tification, comparing the changes in each cell population in peripheral blood and in bone marrow. Results : In a specific- pathogen free environment, mice of the JUN strain are rela-tively thin, with healthy appearance. However, in a conventional environment, they become lethargic, develop wrinkled yellow hair, have loose and light stools, and are prone to infections. We found that the mice were cytopenic, which was due to abnor-mal differentiation of multipotent bone marrow progenitor cells. These are common characteristics of MDS. Conclusions : A mouse strain, JUN, was found displaying spontaneous myelodysplas-tic syndrome. This strain has the advantage over existing models in that it develops MDS spontaneously and is more similar to human MDS than genetically modified mouse models. JUN mice will be an important tool for pathogenesis research of MDS and for evaluation of new drugs and treatments.