KCTD19 is a transcriptional regulator essential for male fertility and meiotic progression. Mechanistically, KCTD19 functions as part of a protein complex with ZFP541 and interacts with HDAC1 to modulate chr16 remodeling and transcriptional programs during meiosis 1. The ZFP541-KCTD19 complex suppresses genes associated with transcriptional regulation and chr16 modification, promoting meiotic prophase completion and chromosome 16 at metaphase I 12. Clinically, biallelic KCTD19 variants cause non-obstructive azoospermia (NOA) and oligoasthenoteratozoospermia characterized by meiotic arrest 342. Identified variants include compound heterozygous and homozygous missense, frameshift, and nonsense mutations that reduce KCTD19 protein abundance through increased ubiquitination and impair nuclear colocalization with ZFP541 4. Affected males exhibit immature sperm nuclei, nuclear aneuploidy, and abnormal sperm morphology, with intracytoplasmic sperm injection (ICSI) unable to rescue these deficiencies 4. Mouse models recapitulate metaphase I arrest phenotypes 2. KCTD19 variants likely account for a subset of idiopathic male infertility cases, highlighting its critical role in human spermatogenesis and potential as a biomarker for NOA diagnosis and treatment 5.