Hi everybody,

I have some nucleotide sequence (fasta format) that contains many sequences with invalid characters, like 'e', 'q', 'i' and 'l', there is also a item named "unavailable sequence" for some header. These characters were reported me as a error during working with MEME-chip. Could anybody please let me know how to find these characters and also the item "unavailable sequence" and remove them? Any suggestion and command warmly welcomed.
cleanup fasta:

    sed -e '/^[^>]/s/[^ATGCatgc]/N/g' file.fa