Heavy and light chain Ig variable region nucleotide sequences, and their respective translated amino acid sequences were obtained from equine fetal (liver and bone marrow) and adult horse (bone marrow) whole tissues. Amino acid variability at each residue of the Ig variable region was calculated as describe by Wu and Kabat (1970) for (a) fetal liver heavy chain, (b) fetal bone marrow heavy chain, (c) adult horse bone marrow heavy chain, (d) fetal liver lambda light chain, (e) fetal bone marrow lambda light chain, and (f) adult horse bone marrow lambda light chain. The three CDR regions are underlined bellow the X-axis.