 Responses to what Burian has labelled “Lillie’s paradox” hinge on differences in gene expression among cells and tissues. Chromosomal DNA is a linear template for an mRNA transcript, which is in turn a template for a sequence of amino acids making up a protein. Cell phenotype depends on which genes are transcribed and then translated. But DNA does not express itself. The challenge then is to account for gene expression in a way that preserves genetic control of development. The concept of a “genetic program for development” responds to this challenge.