SPAG8 (sperm-associated antigen 8) is a testis-specific protein with multiple functional roles in reproductive biology and cell cycle regulation. As a microtubule inner protein component of dynein-decorated doublet microtubules in the ciliary axoneme, SPAG8 is essential for motile cilia beating 1. During spermatogenesis, SPAG8 functions as a novel regulator of the CREM-ACT transcriptional complex, enhancing ACT binding to CREMtau and thereby promoting transcriptional activation of genes critical for germ cell differentiation 2. SPAG8 also regulates cell cycle progression by prolonging the G2/M phase through modulation of cdc2 (CDK1) phosphorylation at Tyr15, potentially delaying mitotic entry during spermatogenesis 3. Clinically, SPAG8 variants and expression levels are associated with nonobstructive azoospermia (male infertility) 4. Additionally, SPAG8 has emerged as a cancer-testis antigen with elevated expression in various tumors (10-70% frequency) and elicits spontaneous humoral immune responses in cancer patients, positioning it as a potential immunotherapeutic target and serologic biomarker 5. These diverse functions underscore SPAG8's importance in both normal reproductive physiology and disease pathogenesis.