What 23andMe offered was a $99 test for some 250 genetically linked conditions, based on a partial reading of single nucleotide polymorphisms (SNPs) – points where individuals’ genomes vary by a single DNA base pair. Given that DTC genetic tests target only a fraction of the human genome’s three billion markers, and various companies sample different SNPs, different tests may return disparate results for the same customer, who might then make serious medical decisions based on inaccurate information.