It looks like Biostrings function "writeFASTA" overwrites the output file at each run. It seems it does not support the "append" parameter. I have to generate one big file gathering a miRNA identifier and relative sequence followd by a variable number of dara records pertaining such a miRNA target genes transcription. Each record is made up of a 3'utr identifier <hgnc_id>.<ensembl_id>.<transript_id> followed by the relative 3'UTR sequence. I hoped I could use writeFASTA but it does not allow me to append sequences to the same output file. Any suggestion is more than welcome. Thank you in advance, Maura .html?pmk=footer> [[alternative HTML version deleted]]