The proposed research seeks to identify molecules whose expression is essential in cells that constitute the stem cell niche in the hematopoietic microenvironment. We are using stromal cell lines that have been shown to be highly supportive for stem cells as surrogates for the in vivo niche. The gene profiles of these stromal cell lines which are from different developmental and tissue origin will be compared with those of less-supportive lines by microarray analyses and verified by quantitative real time PCR. Candidate genes that correlate with a stem cell supporting phenotype will be analyzed by bioinformatic tools for functional designation and predicted cellular role. A selected set of gene products will be investigated further with gene knock-down and over expression methodologies. This study will contribute to our understanding of the molecular mechanisms that govern the hematopoietic stem cell niche and perhaps stem cell niches in general.