HBS1¹ homozygous males show grossly normal testis morphology but lack sperm in the seminal vesicle and the seminal vesicles are much smaller than heterozygous controls; spermatogonia and spermatocytes show comparable sizes to controls, but spermatid cysts frequently show abnormal ratios of nebenkern to nucleus ( 1:1 , 1:2 , 1:3 to 1:4 , instead of the expected 1:1 ratio), abnormal large nebenkern and small or micronuclei, which indicates failure of cytokinesis and chromosome mis-segregation during spermatogenesis; spindle microtubules frequently fail to converge into the poles and are splayed outward; there are defects in later spermatogenesis, as individualization complexes are absent and the spermatid nuclei are randomly scattered along the testicular tube, as compared to controls; there are no change in the numbers of germline stem cells and spermatogonial cells (Bam-GFP positive cells), as compared to controls.

HBS1¹ heterozygotes show a similar number of individualization complexes to wild-type controls.

HBS1¹/HBS1¹⁷² transheterozygous males also have late spermatogenesis defects, namely significantly fewer individualization complexes and randomly scattered spermatid nuclei along the testicular tube, as compared to controls.