A Mutagenesis Screen Performed on Mice Identified a Gene with an Important

A mutagenesis screen performed on mice identified a gene with an important function in B cells. Analysis of B cells from the spleens of these mutant mice showed the results shown in Figure. However, when the C $\delta$ coding sequences were determined, no mutations in these DNA sequences were found. To study this further, genetic crosses were set-up with another mouse strain carrying an allelic variant of the immunoglobulin heavy chain locus. The original mutant mouse strain (strain A) carries IgM^a and IgD^a alleles of these heavy chain genes. The second mouse strain (strain B) carries the IgM^b and IgD^b alleles of the heavy chain genes. These two parental strains were crossed together, generating progeny that were all heterozygous for IgM and IgD alleles B were obtained.

Based on these data, the mutation in the original mutant mouse strain most likely inactivates:

A) The gene for a transcription factor required for IgD gene transcription
B) A regulatory region in the immunoglobulin heavy chain gene locus required for IgD gene transcription
C) The gene for a factor required for alternative mRNA splicing of the immunoglobulin heavy chain primary mRNA transcript
D) The gene encoding the RAG-1 or RAG-2 recombinase
E) The gene for a factor that is required for IgD surface expression