Victor, We too occasionally experienced similar problems. These bands have been eliminiated (so far) by using KCl free buffers for the reactions \\\\ [A[A[A[A[A[A[A[A [B[B[B[B[B[B[B[B[B[B[B[BSee Nucl. Acids Rres. (1995) vol 23 (3). Raj. (mbro at seqnet.dl.ac.uk)