When the cloning era arrived, our first target for cloning was the delta1-crystallin gene of the chicken, the lens-specific gene expressed earliest following lens induction. We have investigated the regulation of this gene with the idea that the mechanism of its activation must reflect that of lens differentiation per se. We here summarize the investigation carried out in our group along this line over the past 20 years. The delta1-crystallin gene is regulated by an enhancer in the third intron, and the specificity of this regulation is governed by a DNA region (called DC5) of only 30 bp DNA bound by two transcription factors. These factors have been identified as SOX1/2/3 (Group B1 SOX proteins, SOX2 being the major player) and Pax6, and have been shown to bind cooperatively to DC5 and form a ternary complex having a robust potency for transcriptional activation. In the embryo, Pax6 is widely expressed in the head ectoderm before the lens is formed, and as the optic vesicle comes into contact with the ectoderm, SOX2/3 expression is induced in the contacted area of the ectoderm, thereby allowing Pax6 and SOX2/3 to meet in the same cell nucleus, where they can then activate a battery of genes for early lens development including delta1-crystallin. Thus, the cooperative action of Pax6 and SOX2 initiates lens differentiation. More broadly, SOX1/2/3 interact with various partner transcription factors, and participate in defining distinct cell states that depend on the partner factors: Pax6 for lens differentiation, Oct3/4 for establishing the epiblast/ES cell state, and Brn2 for the neural primordia. Thus, the regulation of SOX2 (and SOX1/3) and its partner factors, exemplified by Pax6, determines the spatio-temporal order of the occurrence of cell differentiation.