An oligonucleotide comprising (a) a double-stranded portion, which double-stranded portion is DNA and 9 to 30 base pairs in length; (b) a loop connecting the 3' end of the first strand of said double-stranded portion with the 5' end of the second strand of said double-stranded portion, said loop comprising, in 5' to 3' direction: (ba) a first DNA portion which is 4 to 20 nucleotides in length; (bb) a non-nucleic acid spacer selected from an oligo-ethyleneglycol spacer and an oligo-methylene spacer; and (bc) a second DNA portion which is 4 to 20 nucleotides in length; wherein said first DNA portion and said second DNA portion are not complementary to each other; (c) a single-stranded overhang at its 5' end, said overhang being 5 to 40 nucleotides in length, wherein (ca) the bond connecting said double-stranded portion with the nucleotide of said overhang which is directly adjacent to said double-stranded portion is cleavable under alkaline conditions; and (cb) said overhang optionally comprises a barcode sequence, said barcode sequence preferably being 5 to 10 nucleotides in length; and optionally (d) within one, more or all of (a), (b) and (c), one, more or all of the following: (da) one or more modified nucleotides; (db) one or more sequences conferring compatibility with nucleic acid sequencing kits, such compatibility being preferably the presence of regions within said oligonucleotide which are complementary to primers comprised in said sequencing kits; and (dc) one or more random bases. (a) a double-stranded portion, which double-stranded portion is DNA and 9 to 30 base pairs in length; (b) a loop connecting the 3' end of the first strand of said double-stranded portion with the 5' end of the second strand of said double-stranded portion, said loop comprising, in 5' to 3' direction: (ba) a first DNA portion which is 4 to 20 nucleotides in length; (bb) a non-nucleic acid spacer selected from an oligo-ethyleneglycol spacer and an oligo-methylene spacer; and (bc) a second DNA portion which is 4 to 20 nucleotides in length; wherein said first DNA portion and said second DNA portion are not complementary to each other; (ba) a first DNA portion which is 4 to 20 nucleotides in length; (bb) a non-nucleic acid spacer selected from an oligo-ethyleneglycol spacer and an oligo-methylene spacer; and (bc) a second DNA portion which is 4 to 20 nucleotides in length; wherein said first DNA portion and said second DNA portion are not complementary to each other; (c) a single-stranded overhang at its 5' end, said overhang being 5 to 40 nucleotides in length, wherein (ca) the bond connecting said double-stranded portion with the nucleotide of said overhang which is directly adjacent to said double-stranded portion is cleavable under alkaline conditions; and (cb) said overhang optionally comprises a barcode sequence, said barcode sequence preferably being 5 to 10 nucleotides in length; and optionally (ca) the bond connecting said double-stranded portion with the nucleotide of said overhang which is directly adjacent to said double-stranded portion is cleavable under alkaline conditions; and (cb) said overhang optionally comprises a barcode sequence, said barcode sequence preferably being 5 to 10 nucleotides in length; (d) within one, more or all of (a), (b) and (c), one, more or all of the following: (da) one or more modified nucleotides; (db) one or more sequences conferring compatibility with nucleic acid sequencing kits, such compatibility being preferably the presence of regions within said oligonucleotide which are complementary to primers comprised in said sequencing kits; and (dc) one or more random bases. (da) one or more modified nucleotides; (db) one or more sequences conferring compatibility with nucleic acid sequencing kits, such compatibility being preferably the presence of regions within said oligonucleotide which are complementary to primers comprised in said sequencing kits; and (dc) one or more random bases.