Constitutive expression of the Id1 gene impairs mouse B cell development

The Id proteins are inhibitors of the basic-helix-loop-helix (bHLH) transcription factors. In the B cell lineage, the Id1 and Id2 genes are expressed in pro-B cells and down-regulated during differentiation. To determine the role of bHLH proteins and the significance of down-regulation of Id genes in B cell development, transgenic mice constitutively expressing the Id1 gene were generated. Their phenotype suggests that B cell development is impaired at an early stage. Primarily, these mice have few B220+ IgM+ mature B or B220+ CD43- pre-B cells in the bone marrow, reduced frequencies of V(D)J and V kappa J kappa recombination of the immunoglobulin loci, and lower expression levels of the immunoglobulin, RAG-1, RAG-2, and lambda 5 genes.