Molecular Regulation of Lymphoid Cell Growth and Development 
ferences in the generation of B cells in mammals 
and birds to shed light on mammalian B cell 
production. 
Our studies have helped to demonstrate that 
the primary molecular mechanism by which 
chickens generate a wide variety of antibody mol- 
ecules is different from that used by mammals. 
Antibody diversity in the chicken is achieved by a 
process known as gene conversion, which occurs 
during B cell development in the bursa of Fabri- 
cius. Gene conversion requires that the immature 
B cell migrate to the bursa, and that the mature B 
cell, having undergone conversion, emigrate to 
the peripheral lymphoid organs. Understanding 
how this migration is controlled in a regulated 
fashion should lead to insights into how cells 
in a multicellular organism migrate during 
development. 
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