How do we get VCF's ALT field values?

Question

[2012secondseason]

Hi. When I use bcftools only with input BCF file, VCF file involving ALT filed values is created. I am wondering bcftools has autonomously informations for altarnatice alleles.

Thanks.

[Pierre Lindenbaum]

not clear: a vcf/bcf has de facto some ALT fields.