The B cell population is one of the key components of the adaptive immune system, which protects the host from a tremendous variety of pathogens by producing antibodies. B cells develop from hematopoietic stem cells through a pathway known as B lymphopoiesis. This is a process accompanied by intensive gene expression reprogramming. By the end, genes appropriate for the B lineage are activated and those that are not are continuously repressed. The regulation of lineage gene expression is conferred by a network of transcriptional regulators. Although some key components have been defined, more factors, especially those orchestrating the repression of non-B lineage genes, remain to be identified. Chemically induced mutagenesis is a potent way of identifying genes with critical biological functions. Injection of n-ethyl-n-nitrosourea, a mutagen, has generated a unique point mutation in the mouse Gon4-like (Gon4l) gene that specifically causes a loss of peripheral B cells while maintaining the T cell population. The mutation is therefore named Justy for Just T cells. The goal of this thesis project is to analyze the Justy mice and provide insights into the mechanisms underlying the regulation of B lymphopoiesis. The work presented here demonstrates that the protein encoded by Gon4l is essential for early B lymphopoiesis, which is likely through the repression of non-B lineage genes. Gon4l protein contains conserved domains implicated in transcriptional repression and associates in a complex with the transcriptional repression mediators Yin Yang 1 and Sin3a/HDAC1, after these proteins are transiently expressed in cell lines. When bound to DNA, Gon4l is capable of repressing a nearby promoter and this function correlates with its ability to form a complex. Therefore, these results suggest that Gon4l may function as a transcriptional regulator by employing its associated co-factors in the identified complex. Lastly, a wide spectrum of tumors developed in Justy mice, indicating that Gon4l can also act as a tumor suppressor.