Hello: By using "Chimera" under its environment, I retrieved the 3GBM from internet of PDB. Then "H" and "L" chains were selected and then combined into one model. The formats before and after using Chimera are different: Before and download from PDB: original 3GBM.pdb: -------------------------------------------------------------------------- ATOM 8198 N ILE H 52 21.400 -46.642 -15.670 1.00 34.21 N ATOM 8199 CA ILE H 52 21.907 -48.031 -15.705 1.00 34.50 C ATOM 8200 C ILE H 52 22.650 -48.325 -17.003 1.00 34.35 C ATOM 8201 O ILE H 52 22.192 -47.956 -18.089 1.00 34.71 O ATOM 8202 CB ILE H 52 20.781 -49.070 -15.662 1.00 34.67 C ATOM 8203 CG1 ILE H 52 19.817 -48.868 -14.495 1.00 35.26 C ATOM 8204 CG2 ILE H 52 21.375 -50.440 -15.579 1.00 36.91 C ATOM 8205 CD1 ILE H 52 20.481 -48.373 -13.235 1.00 36.77 C ATOM 8206 N PRO H 52A 23.772 -49.033 -16.926 1.00 34.41 N ATOM 8207 CA PRO H 52A 24.505 -49.283 -18.185 1.00 34.55 C ATOM 8208 C PRO H 52A 23.750 -50.255 -19.109 1.00 34.33 C ATOM 8209 O PRO H 52A 22.874 -51.013 -18.645 1.00 34.29 O ATOM 8210 CB PRO H 52A 25.817 -49.905 -17.709 1.00 34.19 C ATOM 8211 CG PRO H 52A 25.402 -50.687 -16.528 1.00 35.46 C ATOM 8212 CD PRO H 52A 24.336 -49.808 -15.815 1.00 34.63 C -------------------------------------------------------------------------- After using Chimera and saved as ".pdb" file: -------------------------------------------------------------------------- ATOM 371 N ILE H 52 -61.722 -45.711 -51.406 1.00 34.21 N ATOM 372 CA ILE H 52 -62.988 -46.402 -51.737 1.00 34.50 C ATOM 373 C ILE H 52 -63.058 -46.767 -53.215 1.00 34.35 C ATOM 374 O ILE H 52 -62.716 -45.957 -54.082 1.00 34.71 O ATOM 375 CB ILE H 52 -64.219 -45.520 -51.499 1.00 34.67 C ATOM 376 CG1 ILE H 52 -64.272 -44.923 -50.094 1.00 35.26 C ATOM 377 CG2 ILE H 52 -65.457 -46.324 -51.739 1.00 36.91 C ATOM 378 CD1 ILE H 52 -63.739 -45.839 -49.022 1.00 36.77 C ATOM 379 N PRO H 52 -63.548 -47.961 -53.533 1.00 34.41 N ATOM 380 CA PRO H 52 -63.580 -48.321 -54.966 1.00 34.55 C ATOM 381 C PRO H 52 -64.618 -47.494 -55.747 1.00 34.33 C ATOM 382 O PRO H 52 -65.552 -46.927 -55.145 1.00 34.29 O ATOM 383 CB PRO H 52 -63.975 -49.797 -54.948 1.00 34.19 C ATOM 384 CG PRO H 52 -64.891 -49.879 -53.793 1.00 35.46 C ATOM 385 CD PRO H 52 -64.271 -48.943 -52.718 1.00 34.63 C -------------------------------------------------------------------------- The "52A" -> "52", there is no discrimination between "ILE" and "PRO" on the position 52 of H chain. Please let me know, what wrong with my data processing? Thank you a lot. Hsih-Te