Hematopoietic differentiation proceeds through a series of binary decisions where lymphoid and myeloid precursors are specified first, followed by more restricted bi- and monopotent progenitors. Each step involves extinction of the original gene expression program and establishment of a novel gene expression program. This is thought to be accomplished by both antagonistic and synergistic transcription factor interactions that establish cell type specific transcription factor combinations. Recent work from t he host laboratory demonstrated that enforced expression in B cells of either the C/EBPa or C/EBPb transcription factor reprograms these cells into macrophages by a series of paralell and sequential events that require endogenous PU.1. The proposed researc h project aims at dissecting the process of trans-differentiation in this system through analysing the changes that occur both at the level of gene expression and of transcription factor occupacy of target gene promoters. For these studies I will use B cel ls infected with an inducible form of C/EBP which will permit to observe the changes in gene expression over time, using Affymetrix-microarrays and quantitave PCR. To identify C/EBP and PU.1 target genes in B cells and macrophages I will use a "ChIP-on-chi p" approach. Results obtained should lead to a better understanding of the lymphoid and myeloid cell branching process and the molecular events accompanying reprogramming of one differentiated cell type into another.