ITDs are internal tandem duplications that frequently occur in the FLT3 gene locus. These duplications usually happen by duplicating nucleotides in sets of 3 and can vary in length from 3 – 100+. 

Mission Bio’s Tapestri Pipeline incorporates a custom algorithm in parallel to the standard GATK/Haplotype genotype caller to call this variant type.

For more information, see this article.