The review concerns conformational properties, interactions with proteins, and expansion during replication of various DNA molecules containing simple sequence repeats (CAG)_n, (CTG)_n, (CCG)_n, and (CGG)_n. Attention is paid not only to the formed relatively stable intrastrand hairpins, but also to their associations into bimolecular heteroduplexes of the complementary strands which appear to have unusually compact conformations difficult to overcome during replication. It is pointed out that some simple sequence-repeat oligonucleotides are expanded by DNA polymerases even in the absence of any natural DNA. The review indicates that DNA conformations of the simple sequence repeats should be taken into account in the studies of genome function, evolution, and pathogenesis.