In mammals, hibernation is expressed by only a limited number of species, and the molecular mechanisms underlying hibernation are not well understood. Recently, we have found plasma proteins which disappear from blood specifically during hibernation in a mammalian hibernator, the chipmunk. Here, we report the cDNA cloning of these chipmunk hibernation-related proteins, , and analyses of their expression. All three proteins contain a collagen-like domain near the N terminus and are highly homologous to each other. Their mRNAs were detected only in liver in nonhibernating chipmunks, and in hibernating chipmunks, the amounts were reduced to less than 1/10 of those in nonhibernating chipmunks, indicating that HP-20, -25, and -27 mRNA expression is regulated similarly in association with hibernation. Southern blot analyses of the squirrel family with each of chipmunk HP-20, -25, and -27 cDNA revealed that a nonhibernating species (tree squirrel) as well as another hibernating species (ground squirrel) retained the corresponding genes. However, their transcripts were detected only with the hibernating species, and in hibernating ground squirrels, their levels were greatly reduced compared with those in nonhibernating animals, as were the cases with the chipmunk. These observations are the first line of evidence for occurrence of hibernation-associated gene regulation. The results would indicate the commitment of HP-20, -25, and -27 to hibernation and support the idea that genetic controls are involved in mammalian hibernation.Most mammals maintain a high, steady body temperature throughout their adult lives, and only certain small mammals, primarily in the orders Rodentia, Insectivora, and Chiroptera, can undergo hibernation. During hibernation, the body temperatures of mammalian hibernators drop to below 10 or even 50C and metabolic rate is reduced to only a few percent of the euthermic level. Heart rate and breathing rate also fall. However, unlike the lower vertebrate and invertebrate hibernators, mammalian hibernators maintain the thermoregulatory control with a lowered set point during hibernation and retain the capacity to arouse from hypothermia by physiological means.Mammalian hibernation accompanies various physiological changes (21). Several histological and morphological studies reveal that polyglandular involution takes place prior to the onset of hibernation and that endocrine functions are generally depressed during the first half of the hibernation season (20). Also, the lymphoid organs undergo an involution in autumn, and the immune system is depressed during hibernation (17). Recently, through comparison of plasma proteins between hibernating and nonhibernating Asian chipmunks (Tamias asiaticus) by using gel permeation highperformance liquid chromatography, we have found that the 140-kDa fraction is markedly reduced in the hibernating animals (9). Four kinds of proteins (20, 25, 27, respectively]) in this 140-kDa fraction of the nonhiberinating chipmunks are practically absent in the corresponding fra...