•Allelic exclusion: expression of only one allele in a diploid cellè one lymphocyte expresses one antigen receptor (ie, productive rearrangement of  both heavy and light chains).
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•To prevent two alleles of each immunoglobulin from making a productive joint, once an allele has a productive joint, the cell ceases current stage of rearrangement and goes to the next stage.
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 Probability of generating a pre-B cell:
     theoretical: 1/3+(2/3x1/3)=55%
     Actual frequency is lower: V genes contains pseudogenes
•   Halting further rearrangement. Two mechanisms:
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(1).Inactivation of RAGs
  - Phosphorylation  of RAG-2è via the pre-B receptor è degradation
  - mRNA synthesis of RAG-1 and RAG-2 is repressed  
(2).Inaccessibility of recombinase to heavy chain gene segments (later)