The PML/SP100 nuclear bodies (NBs) were first described as discrete subnuclear 
structures containing the SP100 protein. Subsequently, they were shown to 
contain the PML protein which is part of the oncogenic PML-RARalpha hybrid 
produced by the t(15;17) chromosomal translocation characteristic of acute 
promyelocytic leukemia. Yet, the physiological role of these nuclear bodies 
remains unknown. Here, we show that SP100 binds to members of the 
heterochromatin protein 1 (HP1) families of non-histone chromosomal proteins. 
Further, we demonstrate that a naturally occurring splice variant of SP100, here 
called SP100-HMG, is a member of the high mobility group-1 (HMG-1) protein 
family and may thus possess DNA-binding potential. Both HP1 and SP100-HMG 
concentrate in the PML/SP100 NBs, and overexpression of SP100 leads to enhanced 
accumulation of endogenous HP1 in these structures. When bound to a promoter, 
SP100, SP100-HMG and HP1 behave as transcriptional repressors in transfected 
mammalian cells. These observations present molecular evidence for an 
association between the PML/SP100 NBs and the chromatin nuclear compartment. 
They support a model in which the NBs may play a role in certain aspects of 
chromatin dynamics.