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Frequently asked questions

Our Scientific Applications Support team has assembled a list of frequently asked questions to help you find answers quickly. Filter using one or more categories to focus on specific topics, or use the search bar to perform a text search.

What should I do if the IDT online ordering tool says that my Alt-R HDR Donor Block is too complex to order?

For most cases, high complexity means that our system has found a region with either very high GC content or several small repeats that would prevent proper synthesis. For Alt-R™ HDR Donor Block sequences with high complexity, we recommend taking the following steps:

  1. Check your desired insertion for regions of high GC content or repeat elements. If possible, alter any coding regions with complexity issues through codon optimization.
  2. Check the homology arms for GC content and repeat elements and, where possible, alter the homology arm length to avoid problematic regions. While 200-300 bp homology arm lengths are recommended for optimal HDR efficiency, homology arms as short as 100 bp may be used. 
  3. If these approaches do not resolve the complexity issues, contact us at for further assistance with your Alt-R HDR Donor Block design.

*RUO—For research use only. Not for use in diagnostic procedures. Unless otherwise agreed to in writing, IDT does not intend for these products to be used in clinical applications and does not warrant their fitness or suitability for any clinical diagnostic use. Purchaser is solely responsible for all decisions regarding the use of these products and any associated regulatory or legal obligations.