Gametocyte-specific factor 1 (Gtsf1), also known as the computationally obtained undifferentiated and/or embryonic stem cell-specific 110 gene (Cue110). Gtsf1 encodes a 167-amino acid protein that is a member of the Uncharacterized Protein Family 0224. DmGTSF1 is essential for P-element-induced wimpy testis (PIWI)-interacting RNA (piRNA)-mediated transcriptional repression and histone H3K9me3-mediated repression of transposons and their neighboring genes in the ovary. DmGTSF1 interacts with Piwi via its C-terminal tail and the two CHHC zinc-finger motifs within the DmGTSF1 protein are required for its activity. The Gtsf1 gene is essential for spermatogenesis and has been revealed to function in transposon suppression in the mouse testes. GTSF1 transcript expression has been observed to be up-regulated in certain leukemias and in lymphomas.