CYLC2 (cylicin 2) is a testis-specific cytoskeletal protein essential for male fertility. Located on chromosome 9, it is exclusively expressed during spermiogenesis and functions as a structural component of the sperm calyx within the perinuclear theca 1. CYLC2 is a lysine-rich, positively charged protein that acts as an actin-binding protein 2, playing critical roles in establishing proper sperm morphology by facilitating calyx assembly, acrosome attachment to the nuclear envelope, and manchette elongation and disassembly during spermatogenesis 1. Loss of CYLC2 function results in male infertility characterized by decreased epididymal sperm counts, impaired cytoplasm shedding, and severe structural sperm malformations including acrosomal abnormalities and compromised sperm motility 1. In humans, infertile men carrying variants in both CYLC1 and CYLC2 display morphological sperm abnormalities, demonstrating the gene's clinical relevance to human male fertility 1. Beyond reproductive function, CYLC2 has been identified as a candidate gene associated with familial pancreatic cancer susceptibility in genetic studies 3, and appears in transcriptome analyses of sex determination and spermatogenesis regulation 4, though these associations require further investigation to establish functional significance.