5 6 Source and Description of Sequence Polymorphism: Sequence analysis of the transmembrane coding part of the human erbB-2/HER2 gene in 27 neuroectodermal tumors and blood cells of 58 healthy individuals revealed a nucleotide polymorphism resulting in either GTC (valine) or ATC (isoleucine) at amino acid codon 655. Sequencing was performed on single nucleotide strands from asymmetric PCR of genomic DNA. The results were confirmed by restriction of an independent double-stranded PCR product (148 bp) with Bsm AI (GTCTC; New England Biolabs). Restriction fragments were 135 bp and 13 bp for the valine allele and 13 bp, 34 bp and 101 bp for the isoleucine allele. To exclude cross-contamination, all experiments were performed in specially designed PCR facilities at least twice and always included blank samples.