Of the 20,000 to 30,000 genes now thought to be in the human genome, most cells express only a very small part - which makes sense, a liver cell has little need for the DNA code that makes neurons.��20000��30000��]�A�{�b�Q�{���b�H����]�աA�j�����ӭM��F�A�u�����ּƪ��@����-�o�ϱo�N�q�W���A�x�ӭM�w�ܤֻݭn��D NA�N�X�A�ϯ��g���C

But as almost all cells carry around a full copy ('distribution') of the genome, a system is needed to #ifdef out stuff not needed.��ѩ�X�G�Ҧ����ӭM�i�楪�k������ƥ��] '���t' �^����]�աA�@�Ө�׬O�ݭn�� ifdef�X�Ӫ��F��ä��ݭn�C And that is just how it works.�o�O��~�O��򰵨쪺�C The genetic code is full of #if/#endif statements.��DZK�X�O�R�����p�G/ �� endif���C

This is why 'stem cells' are so hot right now - these cells have the ability to differentiate into everything.�o�N�O������'�F�ӭM'�o������v�Q�A�{�b-�o�DzӭM����O���Ʀ����@���C The code hasn't been #ifdeffed out yet, so to speak.�N�X�èS���Q�� ifdeffed�X�ӡA��i�H�o�򻡡C

Read more�\Ū��h here�o�� to understand DNA through the eyes of a programmer.�n�F�Ѫ�DNA �A�q�L�������{�ǭ�C