Hi all - I'm constructing a wrapper for a tool I have and the input to the tool can be: 1) a single fastq file (single end sample) 2) multiple singled-end fastq files 3) a single paired-end sample 4) multiple paired-end samples. I have cases #1 and #2 handled, however case #3 is presenting a problem. If the user select "Paired-End" sample, I want to restrict them to selecting only a single fastq1 and fastq2 file. If they want to submit multiple paired-end samples I only want to allow them to submit them as a list of dataset pairs. I can get this part to work. I just need to restrict users to selecting only a single fastq1 and fastq2 file when the input_type is Paired-End. My XML is as follows. Any help would be appreciated <!-- Input FastQ file(s) --> <conditional name="input_type"> <param name="input_type_selector" type="select" label="Select input type" help="Select between single and paired fastq data"> <option value="single">Single-End</option> <option value="paired">Paired-End</option> <option value="paired_collection">Paired Collection</option> </param> <when value="single"> <param name="fastq_input1" type="data" format="fastq" label="Select fastq dataset" multiple="true" help="Specify dataset with single end reads" /> </when> <when value="paired"> <param name="fastq_input1" type="data" format="fastq" label="Select fastq dataset" help="Specify dataset with 1st of paired-end reads" /> <param name="fastq_input2" type="data" format="fastq" label="Select fastq dataset" help="Specify dataset with 2nd of paired-end reads" /> </when> <when value="paired_collection"> <param name="fastq_collection" type="data_collection" collection_type="list:paired" label="Select a paired collection" help="See help section for an explanation of dataset collections"/> </when> </conditional>