Protamines are nuclear proteins which are specifically expressed in haploid male 
germ cells. Their replacement of histones and binding to DNA is followed by 
chromatin hypercondensation that protects DNA from negative influences by 
environmental factors. Mammalian sperm contain two types of protamines: PRM1 and 
PRM2. While the proportion of the two protamines is highly variable between 
different species, abnormal ratios within a species are known to be associated 
with male subfertility. Therefore, it is more than likely that correct protamine 
expression represents a kind of chromatin checkpoint during sperm development 
rendering protamines as suitable biomarkers for the estimation of sperm quality. 
This review presents an overview of our current knowledge on protamines 
comparing gene and protein structures between different mammalian species with 
particular consideration given to man, mouse and stallion. At last, recent 
insights into the possible role of inherited sperm histones for early embryo 
development are provided.