In some cases, a user will not have data at a SNP location. There are two possibilities: No call and Not genotyped.
No call: Occasionally, a user's data may not allow us to determine his or her genotype confidently at a particular SNP. It is possible that future review will allow us to call the genotype, but until that time, the data does not appear. In downloaded data, the entry for any uncalled SNPs displays '--' instead of a two-letter genotype.
Not genotyped: 23andMe periodically updates its DNA analysis platform to take advantage of improvements in technology. Customers tested on different versions of the platform will not have been genotyped for variants that are not included on that specific platform. For these customers, results for these variants appear in Browse Raw Data and other features as "Not Genotyped".