rse functions in melanocyte differentiation, growth and survival pathways [@pgen.1002688-Levy1], [@pgen.1002688-Hoek1], [@pgen.1002688-Strub1].

Yin Yang 1 (YY1) is a ubiquitously expressed zinc-finger transcription factor. It can act as transcriptional repressor or activator [@pgen.1002688-Deng1]. The essential role of YY1 in development is underscored by the fact that genetic ablation of *Yy1* in mice resulted in peri-implantation lethality [@pgen.1002688-Donohoe1]. During B cell and oligodendrocyte lineage development, YY1 functions as a pro-differentiation factor [@pgen.1002688-Liu1], [@pgen.1002688-He1]. In mouse spermatogenesis, YY1 is required for maintaining heterochromatin structure integrity [@pgen.1002688-Wu1]. YY1 thus has important functions in several lineages, but given its ubiquitous expression in the majority of tissues, it is not known whether it is able to regulate select genes in a lineage-specific manner. This paper reports a key role for YY1 in melanocytic lineage development and describes how melanocyte-specific functions of YY1 may be directed by its interaction with M-MITF.

Results {#s2}
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YY1 is required for melanocyte development and survival in vivo and in vitro {#s2a}
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To study the function of YY1 in melanocyte development, we generated melanocyte-specific *Yy1* conditional knockout mice (*TyrCre, yy1f/f*) by crossing *yy1flox/flox* mice [@pgen.1002688-Affarel1] with TyrCre mice in which Cre expression is constitutively driven by the melanocyte-specific tyrosinase (Tyr) promoter [@pgen.1002688-Delmas1]. Cre-mediated genetic recombination starts from embryonic day 10.5. *TyrCre, yy1f/f* mice were born at the expected Mendelian ratio. Shortly after birth (P4), *TyrCre, yy1f/f* mice showed profoundly lighter skin pigmentation compared with littermate controls *TyrCre, yy1f/+* ([Figure 1A](#pgen-1002688-g001){ref-type="fig"}, P4). In the first hair cycle (P0--P28), ventral hairs of *TyrCre, yy1f/f* mice w