78. Rag1 null mice are susceptible to infection because:
1. RAG1 is a protein involved in innate immune response
2. RAG1 is involved in gene rearrangement in all somatic cells
3. RAG1 is involved in gene rearrangement in T and B cells
4. RAG1 is involved in NK cell generation

Detailed Explanation:

RAG1 (Recombination Activating Gene 1) is crucial for the rearrangement of gene segments that encode the antigen receptors on T and B cells. In Rag1 null mice, the lack of RAG1 prevents the formation of functional T and B cells, impairing the adaptive immune response and making these mice more susceptible to infections. This is why the absence of RAG1 has such a significant impact on the immune system’s ability to fight off pathogens.

Thus, the correct answer is:
3. RAG1 is involved in gene rearrangement in T and B cells