SNP has its data available for many organisms as a bitfield, which can compactly hold a great deal of information. For humans, you can access SNP_bitfield.bcp.gz in the human subdirectory of the organism_data directory. The specs for SNP_bitfield located in the specs directory of the dbSNP FTP site.

The bit field can be retrieved using the NCBI C++ Toolkit with its feature iterator and annotation selection classes. This would imply that you have developed a C++ program that will read the bit field too. The bit field can also be retrieved from raw XML or ASN.1 dumps taken from Entrez. You can use the PERL programming language to extract the bit field from there without using C++. (11/08/07)

The only way to read the bitfield is to have retrieved it using the NCBI C++ Toolkit with its feature iterator and annotation selection classes as mentioned in the second paragraph of the previous FAQ in this section. This would imply that you have developed a C++ program that will read the bit field too. The bit field can also be retrieved from raw XML or ASN.1 dumps taken from Entrez. You can use the PERL programming language to extract the bit field from there without using C++.

To get "RS numbers, locations, frequencies, and flanking sequence for all SNPs on human chromosome 18", you will need to download two files:

• You will need the genotype report file by chromosome (gt_chr18.xml.gz) located in the /human_9606/genotype/ directory of the dbSNP FTP site. In this file you will find the rs number, chromosome positions and allele frequencies (organized by each member ss and population).
• The best file to download for refSNP flanking sequences is the rs_ch18.fas.gz file located in the /human_9606/rs_fasta/directory of the dbSNP FTP site.

(09/10/08)