Abstract: Determination of a frequency distribution of nucleotide sequence variants—differing from a nucleotide reference by at least one intermediate part—of a sample is disclosed. A reference set of sequence read data is generated by performing a plurality of reference sequence reads (each having a read start site, RSS) for each sequence reference variant of a reference variant list. It is determined (for each RSS and for the intermediate part of each sequence reference variant) whether the intermediate part is detectable by corresponding reference sequence read. A sample set of sequence read data is generated by performing sample sequence reads for the sample, and a total number of occurrences in the sample set is determined for each RSS and for the intermediate part of each sequence reference variant. The frequency distribution of the plurality of nucleotide sequence variants is determined based on the number of occurrences determined for an RSS where the intermediate part is detectable.