The invention relates to the use of morpholino-nucleosides of formula: ##STR00001## in which R.sup.1 represents a nucleic base and R.sup.2 represents a group corresponding to one of the following formulae: --(CH.sub.2).sub.n--NH.sub.2 --(CH.sub.2).sub.n--SH --(CH.sub.2).sub.n--COOH --(CH.sub.2).sub.n--OH --(CH.sub.2).sub.n--NH--R.sup.3 --(CH.sub.2).sub.n--SR.sup.3 --(CH.sub.2).sub.n--CO--R.sup.3 --(CH.sub.2).sub.n--OR.sup.3 in which n is an interger ranging from 1 to 12 and R.sup.3 is a group derived from a label, from a protein, from an enzyme, from a fatty acid or from a peptide, as chain terminators in a process of DNA or RNA sequencing by the Sanger method, or for the labelling of DNA or RNA fragments.