1136 | METHODS LETTER TO THE EDITOR 1 Division of Cardiology, David Geffen School of Medicine, University of California, Los Angeles, CA, USA 2 Division of Dermatology, Department of Medicine, David Geffen School of Medicine, University of California, Los Angeles, CA, USA 3 Department of Human Genetics, David Geffen School of Medicine, University of California, Los Angeles, CA, USA Correspondence Stephen G. Young, University of California, Los Angeles, Los Angeles, CA, USA. Email: sgyoung@mednet.ucla.edu REFERENCES [1] J. Fischer, B. Bouadjar, R. Heilig, M. Huber, C. Lefèvre, F. Jobard, F. Macari, A. Bakija-Konsuo, F. Ait-Belkacem, J. Weissenbach, M. Lathrop, D. Hohl, J. F. Prud’homme, Hum. Mol. Genet. 2001, 10, 875. [2] O. Adeyo, B. B. Allan, R. H. Barnes 2nd, C. n. Goulbourne, A. Tatar, y. Tu, L. C. young, M. M. Weinstein, P. Tontonoz, L. G. Fong, A. P. Beigneux, S. G. young. J. Invest. Dermatol. 2014, 134, 1589. [3] M. A. Brooke, S. L. Etheridge, n. Kaplan, C. Simpson, E. A. O’Toole, A. Ishida-yamamoto, O. Marches, S. Getsios, D. P. Kelsell, Hum. Mol. Genet. 2014, 23, 4064. [4] B. M. Kenner-Bell, A. S. Paller, M. E. Lacouture, J. Am. Acad. Dermatol. 2010, 63, e58. [5] C. M. Allan, S. Procaccia, D. Tran, y. Tu, R. H. Barnes 2nd, M. Larsson, B. B. Allan, L. C. young, C. Hong, P. Tontonoz, L. G. Fong, S. G. young, A. P. Beigneux, J. Invest. Dermatol. 2016, 136, 436. [6] C. K. Jiang, T. Magnaldo, M. Ohtsuki, I. M. Freedberg, F. Bernerd, M. Blumenberg, Proc. Natl Acad. Sci. USA 1993, 90, 6786. [7] I. Ferby, M. Reschke, O. Kudlacek, P. Knyazev, G. Pantè, K. Amann, W. Sommergruber, n. Kraut, A. Ullrich, R. Fässler, R. Klein, Nat. Med. 2006, 12, 568. [8] n. C. Luetteke, H. K. Phillips, T. H. Qiu, n. G. Copeland, H. S. Earp, n. A. Jenkins, D. C. Lee, Genes Dev. 1994, 8, 399. SUPPORTING INFORMATION Additional Supporting Information may be found online in the supporting information tab for this article. Figure S1 PCR genotyping of Slurp1 and Egfr alleles. We amplified a 377-bp Slurp1 fragment and a 230-bp Egfr fragment from genomic DnA isolated from mouse tail biopsies. To identify mutant alleles, the two differet PCR products were digested with AflII (Slurp1) or FokI (Egfr) and analyzed by agarose gel electrophoresis Figure S2 Failure of homozygosity for the Egfr wa2 allele to amelio- rate the “reduced body weight” phenotype associated Slurp1 defi- ciency. Male mice [Slurp1 +/+ ;Egfr +/+ (n=11), Slurp1 −/− ;Egfr +/+ (n=10), Slurp1 +/+ ;Egfr wa2/wa2 (n=8), Slurp1 −/− ;Egfr wa2/wa2 (n=6)] were weighed weekly from 4 to 16 weeks of age. Graph depicts mean±SEM. *At 16 weeks of age, body weights were lower in Slurp1 −/− ;Egfr +/+ (P=.0201) and Slurp1 −/− ;Egfr wa2/wa2 mice (P=.0022) than in Slurp1 +/+ ;Egfr +/+ mice Accepted: 11 April 2017 DOI: 10.1111/exd.13358 METHODS LETTER TO THE EDITOR An efficient method for gene knock-down by RNA interference in human skin mast cells Abstract Mast cells (MCs) from human skin have been notoriously resistant to gene manipulation, and a method to knock-down gene expres- sion in in situ differentiated MCs is highly desired. The Dharmacon Accell ® transfection system proved successful on several “difficult-to- transfect” cells. In the present work, we therefore tested this method on skin-derived MCs using different siRnA entities. The siRnA was readily taken up, followed by pronounced, specific reduction of gene and protein expression. Hence, we present the first efficient technique for the manipulation of gene expression in primary skin MCs ex vivo, which combines high transfection rates with retained cell viability. 1 | BACKGROUND RnA interference (RnAi) has been firmly established to aid in the dissection of gene function. However, utility of this type of targeted knock-down critically depends on an efficient and non- toxic delivery method of the siRnA into the target cell, and this may be difficult to achieve with primary cells. The Accell ® sys- tem (Dharmacon Accell ® siRnA; GE Healthcare Dharmacon, CO, USA) uses chemically modified naked siRnAs for “self-delivery”, that is without necessity of a transfection reagent (http://dhar- macon.gelifesciences.com/rnai-and-custom-rna-synthesis/sirna/ Accell-sirna/).