A source of microsatellite polymorphism. During DNA replication, the polymerase periodically dissociates from the site of synthesis of the lagging strand. If this occurs in a microsatellite sequence, a region of dinucleotide repeats, the 5'-end of the lagging strand may momentarily dissociate from the template and incorrectly reassociate with a dinucleotide loop in the newly synthesised strand or in the template. When the polymerase complex reassociates and continues synthesis, an extra dinucleotide may be inserted if the loop had been in the lagging strand, or deleted, if the loop had been in the template. When amplified and electrophoresed, sequences with such polymorphism display shadow or stutter bands that differ by 2 bp.