TNP1 (transition protein 1) is a highly basic nuclear protein of 54 amino acids 1 that plays a critical role in replacing histones with protamines during mammalian spermatogenesis. TNP1 is expressed in haploid spermatogenic cells during spermiogenesis, where it promotes the recruitment and processing of protamines responsible for histone eviction and sperm chr2 condensation 2. The TNP1 gene, located on chromosome 2-q36 1, contains a single intron and is regulated by TATA boxes and cAMP-response elements (CREs) with conserved 5' and 3' untranslated regions across mammalian species 3. TNP1 expression is coordinated with the histone-to-protamine transition through phase-separated condensates involving regulatory proteins like CCER1 2. Clinically, TNP1 genetic variations are associated with male infertility: a 15-nucleotide deletion in the TNP1 promoter region reduces expression and causes infertility 4, while the TNP1 GCG haplotype significantly increases azoospermia risk 5. TNP1 serves as a marker for spermatid differentiation in testicular organoids 6. Additionally, TNP1 functions as an autoantigen in lupus nephritis, with anti-TNP1 antibodies detected in 63.6% of lupus nephritis patients 7.