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�������ڱ���Ϊ����������飬�󲿷�ϸ���ֻ�м������һ����-��ʹ��������˵����ϸ���Ѻ�����Ҫ��D NA���룬ʹ��Ԫ��

But as almost all cells carry around a full copy ('distribution') of the genome, a system is needed to #ifdef out stuff not needed.�����ڼ������е�ϸ��������ҵ������� '����' ���Ļ����飬һ���ƶ�����Ҫ�� ifdef�����Ķ���������Ҫ�� And that is just how it works.���Ǹղ�����ô�����ġ� The genetic code is full of #if/#endif statements.�Ŵ������dz������/ �� endif���?

This is why 'stem cells' are so hot right now - these cells have the ability to differentiate into everything.�����Ϊʲô'��ϸ��'��ô�ȵ�Ȩ������-��Щϸ���������ֻ��ɵ�һ�С� The code hasn't been #ifdeffed out yet, so to speak.���벢û�б��� ifdeffed��������������ô˵��

Read more�Ķ���� here���� to understand DNA through the eyes of a programmer.Ҫ�˽��DNA ��ͨ�����еij���Ա��