The most important of these difficulties is that in some formats, such as the PDB format, residues are identified with numbers, while TNT's format uses the full alphanumeric character set in residue names. While the conversion from PDB format to ATOMC is quite simple in this respect the reverse operation is not clearly defined. The solution used by Convert is to simply write the original residue names in the PDB file. This choice easily leads to illegal PDB files.