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Need a library of related DNA or RNA oligo sequences?

Even nonstandard, modified bases can be used in custom DNA or RNA products

Product spotlight: Build variability into your oligo sequences by incorporating Mixed Bases. We offer mixes of multiple base types as well as nonstandard and modified bases.

Quick facts:

Availability: DNA or RNA Location: 5', internal, and 3'
Scales: 25 nmol to large scale
Purification: Standard desalt
Odering: Please email all requests for non-catalog modified bases to for review and quoting

Randomized sections of sequence

IDT offers oligonucleotides containing randomized, or "Mixed Bases". The final order contains a population of oligos with a different base at the randomized position(s). Mixed Bases are ideal for creating pools of oligos that include matches to a variable or unknown template sequence. Mixed Bases can also be used to create diversity in clone libraries and in site-directed mutagenesis experiments.

IDT offers two types of randomization, standard and custom mixed bases. Standard mixed bases can be made at any/ all base sites at no additional charge. At these base positions, the synthesizer pulls an equal ratio of the desired bases. Standard mixed bases will potentially have variation from the designated ratio in the final oligo population.

Custom mixed bases use customer-defined ratios for the desired positions. Pricing for custom mixed bases is calculated by the number of unique ratios per order with a maximum of 4 distinct custom formulations per oligonucleotide. When entering your sequence, use the Bases tab in the Sequence entry portion of the Oligo entry tool. It lists the IUPAC-IUB symbols and is where custom mix ratios should be entered (see Figure 1). Custom services include additional fees in addition to the standard oligonucleotide synthesis cost. For more information on ordering formats, please go to our mixed bases webpage.

Ordering tool for standard and custom mixed bases at IDT.

Mixes of nonstandard and modified bases

In addition to IDT standard and custom mixed bases options for randomizing bases, we offer multiple base types (e.g., DNA, RNA, Affinity Plus/locked nucleic acid, or 2'-0-methyl). This allows the incorporation of multiple base types or modifications from the IDT list of modified base offering into a single position on an oligo. For example, an oligo can have a single position synthesized with 50% T Affinity Plus (locked nucleic acid) and 50% G Affinity Plus base modification. Custom oligos can also contain some nucleotide positions that are mixed bases with standard DNA and other positions with modified bases. Check the mixed bases page for helpful hints for ordering these customizations.

Please note that nonstandard mixed bases are considered custom and are subjected to the maximum of 4 distinct custom formulations per oligonucleotide. Custom services include additional fees in addition to the standard costs for the oligo synthesis.

Submitting requests for non-catalog modifications

IDT scientific experts meet regularly to review the complexity and feasibility of producing non-catalog modifications.

If the modification you need is not listed on the Oligo modifications page, please submit a request for review, email with your name, organization, and sequence designs. Custom services include additional fees in addition to the standard costs for oligo synthesis and/or purifications.

Published Jan 29, 2016
Revised/updated Feb 20, 2020