I like a Perl solution: 
#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my @result = ( '' );

my @plan = (
  [ qw/ CAT CAC / ],
  [ qw/ TGG / ],
  [ qw/ GTT GTC GTA GTG / ],
  [ qw/ CCT CCC CCA CCG / ],
  );

for my $step ( @plan )
  {
  my @new_result;
  for my $suffix ( @$step )
    {
    push @new_result, map { "$_$suffix" } @result;
    }
  @result = @new_result;
  }

print Dumper(\@result);

[download]
though I was surprised that bash's (4.1.2) echo would do what you want: 
#!/bin/bash

echo {CAT,CAC}TGG{GTT,GTC,GTA,GTG}{CCT,CCC,CCA,CCG}

[download]

In reply to Re: System Command using Bash Expansion by eye
in thread System Command using Bash Expansion by azheid

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