subtype 'DNAString'
  => as Str
  => where { $_ && $_=~/^[AGCTN]+$/i}
  => message {"Not a DNA string: only AGCT or N allowed and cannot be empty"};

##</code><code>##

use MyApp::MooseX::MyTypes:
has sequence => (
  isa      => 'DNAString',
);

##</code><code>##

coerce 'DNAString'
  => from duck_type( ['seq'] )
  => via { $_->seq };

##</code><code>##

use MyApp::MooseX::MyTypes:
has sequence => (
  isa      => 'DNAString',
  coerce   => 1, 
);