Answer: The differences can originate from two potential sources or combination of the two: 1) naturally occurring events including polymorphisms, alternative splicings, inconsistencies in multiple copies/versions of the same genes resulting from annotation updates, sequencing errors and/or ambiguities; 2) artificially introduced changes by PCR cloning process. Very often, potential users of our clones compare our sequencing data with the sequence of only one reference transcript, and attribute any discrepancies to PCR introduced mutations. Through analysis of multiple clones of close to 20,000 human genes, we found that vast majority of discrepancies can be clearly attributed to category 1), when comparing our sequence data with all transcripts and sequences in corresponding gene locus including genomic sequence, ESTs, other cDNA/mRNA transcripts. On average, we found that there are fewer discrepancies between the sequences of our ORF clones and corresponding genomic sequence than the sequences of public domain gene transcripts and corresponding genomic sequences. We certainly cannot entirely eliminate the possibility of mutations introduced by our PCR cloning process, but we believe they are rare occurrences. We offer services at a small fee for changing base nucleotide compositions using a proprietary non-PCR based mutagenesis kit.