 post-developmental survival in vivo.

![YY1 is required for melanocyte development in vivo.\
(A) Skin and hair pigmentation phenotype of *TyrCre, yy1f/fl* in the first (P4, P10) and second (P45) hair cycles. (B) H&E staining of hair follicles from skin sections of *TyrCre, yy1f/+* and *TyrCre, yy1f/f* mice in the first (P4) and second (P38) hair cycles. Arrows point to the hair follicles still containing pigment. (C,D) Immunofluorescence staining of DCT (red) in P4 hair follicles of *TyrCre, yy1f/+* (C) and *TyrCre, yy1f/f* (D) mice. Skin sections were stained with goat anti-DCT primary antibody and donkey anti-goat Alexa 594 secondary antibody. Nuclei were counterstained with DAPI (blue). (E,F) Immunofluorescence staining of DCT (red) and YY1 (green) in P4 hair follicles of *TyrCre, yy1f/+* (E) and *TyrCre, yy1f/f* (F) mice. Skin sections were double-stained with goat anti-DCT and rabbit anti-YY1 primary antibodies and then donkey anti-goat Alexa 594 and goat anti-rabbit Alexa 488 secondary antibodies. (G,H) Zoom-in view of the dashed box area in (F). DAPI stain is in blue in (H). (I,J) Immunofluorescence staining of DCT (red) in P38 hair follicles of *TyrCre, yy1f/+* (I) and *TyrCre, yy1f/f* (J) mice. Nuclei were stained with DAPI (blue). (K&L) XGal stain of whole-mount skin sections from P38 *TyrCre, yy1f/+, Dct-LacZ* (K) and *TyrCre, yy1f/f, Dct-LacZ* (L) mice.](pgen.1002688.g001){#pgen-1002688-g001}

To determine whether YY1 is also required for melanocytic cell survival in vitro, we stably knocked down endogenous YY1 using two lentiviral shRNAs (yy1shR\#1&\#6) ([Figure 2A](#pgen-1002688-g002){ref-type="fig"}). Prolonged depletion of YY1 led to decreased cell numbers in both YY1 knockdown MALME-3M melanoma cells and human foreskin primary melanocytes (HFM), similar to MITF knockdown ([Figure 2B and 2C](#pgen-1002688-g002){ref-type="fig"}, [Figure S2A](#pgen.1002688.s002){ref-type="supplementary-material"}). While a decrease in cell numbers clearly indicates cell death, it is plausible that there could also 