This protein family is restricted to a subset of endospore-forming bacteria such as Bacillus subtilis, all of which are in the Firmicutes (low-GC Gram-positive) lineage. It is a minor SASP (small, acid-soluble spore protein) designated SspK. [Cellular processes, Sporulation and germination]
Jiyao W et al.(2023). "The conserved domain database in 2023.",