Normal bone marrow function depends on the coexistence of normal hematopoietic stem cells and a microenvironment mostly located in the medullary part of the bones. Stem cells cannot function properly in the absence of an adequate microenvironment. Whereas in malignant and non-malignant hematologic diseases caused by a deficiency or abnormal stem cells, stem cell transplantation is the treatment of choice that results in a cure, in diseases such as MDS, myelofibrosis, and other conditions associated with an abnormal microenvironment, pancytopenia may occur despite the presence of apparently normal hematopoietic cells with no recognizable cytogenetic abnormality.