FLT3LG and neutropenia: [21] In diseases affecting only single blood lineages, an increase in FL is not seen. [22] Though FL mRNA is ubiquitously expressed throughout the body, the protein has only been found on T lymphocytes and stromal fibroblasts within the bone marrow microenvironment. [21], [22], 23 Exposure to radiation of the hematopoietic stem cell niche also results in an increase to FL expression. This has been seen in patients undergoing radiation therapy with simultaneous onset of leucopenia, neutropenia and thrombocytopenia. [9], [10], [24].