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A schematic diagram of normal human spermatozoa. The black box represents the sperm HTCA, and defects in which may cause ASS. The marked parts are what we known and others need us to explore further. ASS: acephalic spermatozoa syndrome; SUN5: Sad1 and UNC84 domain containing 5; PMFBP1: polyamine-modulated factor 1 binding protein 1; SPATA6: spermatogenesis associated 6; HTCA: head–tail coupling apparatus.
