Hsf5 KO male mice exhibit abnormalities in sex chromosome synapses. A) Synapsis analyses of spermatocytes from WT and Hsf5 KO mice with SYCP3 and SYCP1 by immunostaining surface‐spread spermatocytes. Hsf5 KO mice exhibited abnormal sex chromosome synapsis. Quantification of X‐Y normal chromosome synapsis per nucleus in pachytene stage (N = 3 biologically independent WT mice and KO mice; n, the total number of nuclei analyzed; two‐sided Student's t test; *P < 0.05; error bars, mean ± SEM; green, SYCP1; red, SYCP3; scale bars, 10 µm). B) The magnified panels showed abnormally synapsed sex chromosomes (Green, SYCP1; red, SYCP3; scale bars, 10 µm). C) Chromosome spreads of WT and Hsf5 KO spermatocytes were stained for SYCP3 and HORMAD1. Quantification of non‐PAR X‐Y matching per nucleus in pachynema spermatocytes (N = 3 biologically independent WT mice and KO mice; n, the total number of nuclei analyzed; two‐sided Student's t test; *P < 0.05; error bars, mean ± SEM; green, HORMAD1; red, SYCP3; scale bars, 10 µm). The white arrow indicates the X chromosome, and the blue arrow indicates the Y chromosome.