def get_sra_ftppath(srrid):
    """Return an ftp address to download sra files
    """
    if len(srrid) <6:
        return None
    
    parent_path="anonftp@ftp.ncbi.nlm.nih.gov:/sra/sra-instant/reads/ByRun/sra/"
    parent_path=parent_path+srrid[0:3]+"/"+srrid[0:6]+"/"+srrid+"/"+srrid+".sra"
    
    return parent_path