PFN4 (profilin family member 4) is a testis-specific protein essential for male fertility, functioning distinctly from somatic profilin isoforms. Unlike canonical profilins, PFN4 does not bind actin or poly-L-proline in vitro; instead, it selectively binds phosphoinositides including PtdIns(3)P, PtdIns(4,5)P2, PtdIns(4)P, and phosphatidic acid 1. During spermiogenesis, PFN4 localizes to the acroplaxome-manchette complex and is required for manchette development and acrosome biogenesis 2. PFN4-deficient mice produce sperm with severely impaired manchette formation, disrupted microtubular organization, and defective acrosome development due to dysregulation of Golgi membrane trafficking and altered PI3K/AKT/mTOR signaling 2. These ultrastructural defects result in reduced sperm motility and amorphous head shape 2. Notably, PFN4-deficient sperm retain fertilization capacity in zona-free conditions, suggesting potential therapeutic interventions for PFN4-related human infertility 2. Single-cell chr2 accessibility analysis reveals unique regulatory regions upstream of PFN4 that are highly accessible during human spermatogenesis, underscoring its importance in male germ cell development 3.