Primary lymphedema is caused by anatomic or functional defects in the lymphatic system, resulting in chronic swelling of body parts. There may be accompanying nail and skin changes, such as nail dysplasia or papillomatosis. Onset is usually at birth or in early childhood but can occur later, and the severity is variable (summary by Gordon et al., 2013 and Balboa-Beltran et al., 2014). Genetic Heterogeneity of Lymphatic Malformation Primary lymphedema is genetically heterogeneous: see also LMPHM2 (611944), which maps to chromosome 6q16.2-q22.1; LMPHM3 (613480), caused by mutation in the GJC2 gene (608803) on chromosome 1q42; LMPHM4 (615907), caused by mutation in the VEGFC gene (601528) on chromosome 4q34; LMPHM5 (153200); LMPHM6 (616843), caused by mutation in the PIEZO1 gene (611184) on chromosome 16q24; LMPHM7 (617300), caused by mutation in the EPHB4 gene (600011) on chromosome 7q22; LMPHM8 (618773), caused by mutation in the CALCRL gene (114190) on chromosome 2q31; LMPHM9 (619319), caused by mutation in the CELSR1 gene (604523) on chromosome 22q13; LMPHM10 (610369), caused by mutation in the ANGPT2 gene (601922) on chromosome 8p23; LMPHM11 (619401), caused by mutation in the TIE1 gene (600222) on chromosome 1p34; LMPHM12 (620014), caused by mutation in the MDFIC gene (614511) on chromosome 7q31; LMPHM13 (620244), caused by mutation in the THSD1 gene (616821) on chromosome 13q14; and LMPHM14 (620602), caused by mutation in the ERG gene (165080) on chromosome 21q22. Lymphedema can also be a feature of syndromic disorders such as lymphedema-distichiasis syndrome (153400), which is caused by mutation in the FOXC2 gene (602402), and various forms of nonimmune hydrops fetalis (NIHF; see 236750).