Testis-specific histone H1 that forms less compacted chromatin compared to other H1 histone subtypes. Formation of more relaxed chromatin may be required to promote chromatin architecture required for proper chromosome regulation during meiosis, such as homologous recombination. Histones H1 act as linkers that bind to nucleosomes and compact polynucleosomes into a higher-order chromatin configuration. Testis-specific. This histone is a testis-specific H1 variant that appears during meiosis in spermatogenesis. Phosphorylated in early spermatids. Citrullination at Arg-57 (H1R54ci) by PADI4 takes place within the DNA-binding site of H1 and results in its displacement from chromatin and global chromatin decondensation, thereby promoting pluripotency and stem cell maintenance. Belongs to the histone H1/H5 family.