HENMT1 (HEN methyltransferase 1) is a small RNA 2'-O-methyltransferase that catalyzes 2'-O-methylation at the 3' end of piRNAs (PIWI-interacting RNAs), 24-30 nucleotide germline-specific small RNAs 1. This methylation stabilizes piRNAs by protecting them from degradation via uridylation and terminal nucleotidyl transferase-mediated tailing 2, which is essential for piRNA function. Beyond piRNAs, HENMT1 also methylates 3'-tRNA fragments (3'-tRFs), protecting them from degradation and enabling their role in endogenous retrovirus silencing 3. HENMT1 is critically required for transposon silencing in germ cells. In adult spermatogenesis, HENMT1 loss results in piRNA instability, transposable element de-repression, DNA damage, and male sterility characterized by germ cell arrest at the elongating spermatocyte phase 1. HENMT1 and PIWIL1 are coexpressed in pachytene spermatocytes and spermatids, with samples expressing HENMT1 showing significantly lower transposon expression 4. Clinically, homozygous loss-of-function variants in HENMT1 cause non-obstructive azoospermia (NOA) with severe male infertility 52. A frameshift variant (c.47_75del) resulted in reduced piRNA abundance and azoospermia 2. These findings establish HENMT1 as essential for maintaining germline integrity and spermatogenic competency through piRNA-mediated transposon silencing.