The CH1 domains of antibodies belonging to the following five murine 
immunoglobulin (Ig) classes IgG1, IgG2a, IgG2b, IgG3 and IgA have been compared. 
The IgG CH1 domain structures are, as would be expected, similar overall, but 
show local conformational variations. When compared with IgG CH1 domain 
structures, the IgA CH1 domain displays several significant structural 
differences, which are a consequence of insertions/ deletions and specific 
structural constraints. In regions of structural differences in the IgG CH1 
domains, the spatial correspondence of residues is not reflected by conventional 
(Kabat) sequence number. Thus the sequence alignment and numbering for CH1 
domains has been revised to be consistent with the three-dimensional alignments.