Lys H66->Arg:
Most frequently observed mutation, in 48 of total of 81 clones, 2/22 in 1st group, 19/21 in 2nd, 15/16 in 3rd and 12/12 in 4th. Affects buried charge cluster in VH: H66 Arg interacts directly with H86 Asp(highly conserved) and H85 Asp or Glu, which in turn also interacts with R H38

Position H66 is highly conserved in humans, less so in mouse:human: 90.4% R, 1.3% K, ( 2.4% H, 5.2% Q (vh5)); mouse: 47.1% R, 51.4% K

Related Positions:
Position H86 Asp: human: 98.5% D, 0.2% E; mouse 98.8% D
Position H85 Asp: human:11.2% D, 56.2% E, 21.5% A; mouse 10.2% D, 85.6% E
Position H38 Arg: human 97.9%R, mouse 47.8% R, 50.4% K also part of the cluster are Position H43 and H46
Position H43 Arg: human: 66% K, 10% R, 20% Q; mouse 58% K, 6% R, 26% Q
Position H46 Glu: human 94% E, mouse 95%E

It may be interesting to see whether the identity of the different charged residues covariate in a group-specific manner, eg H66K beeing more frequent in conjunction with H85E, and how well abpc48 agrees with its germline sequence

Urea denaturation: K H66 R is markedly more stable than K H3 I, G H75 S, Y L50 H, with a +/- parallel shift of the denaturation curve