In mature B cells of mice and most mammals, cellular release of single H chain 
Abs without L chains is prevented by H chain association with Ig-specific 
chaperons in the endoplasmic reticulum. In precursor B cells, however, surface 
expression of mu-H chain in the absence of surrogate and conventional L chain 
has been identified. Despite this, Ag-specific single H chain Ig repertoires, 
using mu-, gamma-, epsilon-, or alpha-H chains found in conventional Abs, are 
not produced. Moreover, removal of H chain or, separately, L chain 
(kappa/lambda) locus core sequences by gene targeting has prevented B cell 
development. In contrast, H chain-only Abs are produced abundantly in Camelidae 
as H2 IgG without the C(H)1 domain. To test whether H chain Abs can be produced 
in mice, and to investigate how their expression affects B cell development, we 
introduced a rearranged dromedary gamma2a H chain into the mouse germline. The 
dromedary transgene was expressed as a naturally occurring Ag-specific 
disulphide-linked homodimer, which showed that B cell development can be 
instigated by expression of single H chains without L chains. Lymphocyte 
development and B cell proliferation was accomplished despite the absence of L 
chain from the BCR complex. Endogenous Ig could not be detected, although V(D)J 
recombination and IgH/L transcription was unaltered. Furthermore, crossing the 
dromedary H chain mice with mice devoid of all C genes demonstrated without a 
doubt that a H chain-only Ab can facilitate B cell development independent of 
endogenous Ig expression, such as mu- or delta-H chain, at early developmental 
stages.