Phred reads DNA sequencer trace data, calls bases, assigns quality values to the bases, and writes the base calls and quality values to output files. Phred can read trace data from chromatogram files in the SCF, ABI, and ESD formats. It automatically determines the file format, and whether the chromatogram file was compressed using gzip, bzip2, or UNIX compress. After calling bases, phred writes the sequences to files in either FASTA format, the format suitable for XBAP, PHD format, or the SCF format. Quality values for the bases are written to FASTA format files or PHD files, which can be used by the phrap sequence assembly program in order to increase the accuracy of the assembled sequence.