# Can NextSeq 1000/2000 custom primers be diluted in in RSB with Tween instead of HT1? Custom sequencing primers for the NextSeq 1000/2000 must be diluted in HT1, per the [NextSeq 1000/2000 Product Documentation Custom Primers Prepare and Add Custom Primers](https://support-docs.illumina.com/IN/NextSeq10002000/Content/IN/NextSeq2000_1000/PrepareAddCP_intro_fNS_m2000.htm). HT1 must either be [purchased separately](https://www.illumina.com/products/by-type/accessory-products/ht1-buffer.html) (cat. 20015892) or purchased bundled in the [NextSeq 1000/2000 primer kits](https://www.illumina.com/products/by-type/sequencing-kits/cluster-gen-sequencing-reagents/nextseq-1000-2000-reagents.html), listed below. * NextSeq 1000/2000 Read and Index Primers (cat. 20046115) * NextSeq 1000/2000 Index Primer Kit (cat. 20046116) * NextSeq 1000/2000 Read Primer Kit (cat. 20046117) Custom primers diluted in RSB-Tween, rather than HT1, has been observed to cause cycle 1 failures. \ \ \
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