Run STAR on .fastq files I1, R1 and R2


I'm trying to do STAR alignment on 10x data (I tried cellranger but I need a more customizable tool), but I'm a bit confused about the different fastq files and which ones to merge together.
All my samples consists of .gz folders which have multiple files, but they come in triplets such as _S1_L001_I1_001.fastq, _S1_L001_R1_001.fastq and *_S1_L001_R2_001.fastq. Now I understand that R1 and R2 probably refer to the Illumina pair-end reads, but what is I1?

More concretely, which files should be given as arguments to --readFilesIn and in which order? In the manual and some examples I found that R1 and R2 both have to be supplied together, e.g. --readFilesIn *_R1.fastq *_R2.fastq. If I want to align all the reads, do I loop over this command, taking all R1 and R2 files, but ignore the I1 files?





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