Table 1.

Proposed list of attributes for evaluation of individuals with mosaicism; the “A to F” evaluation of mosaicism. See also Fig. 1 and Fig. S1–S6.

Letter	Item to be evaluated	Explanation	Subdivision of the attribute
A	Affected tissue	The part of the body harboring the variant cells which are either, somatic cells, germinal cells, a combination of somatic and germinal cells or not affecting the embryo/neonate but only the placental tissue.	A1. Somatic mosaicism A2. Gonadal (germinal) mosaicism A3. Gonadosomatic mosaicism A4. Confined placental mosaicism A0. If unknown
B	Body pattern	The B of the classification refers the body pattern. This is an anatomic category in which the extent and distribution patterns of mosaic clinical manifestations are classified. We propose two major classes: non-segmental and segmental mosaicisms.	Non-Segmental patterns B1. Single point B2. Disseminated B3. Patchy without midline separation B0. No pattern (e.g., hematologic) Segmental patterns B4. Blaschko lines, narrow bands B5. Blaschko lines, broad bands B6. Checkerboard B7. Phylloid B8. Lateralization/half body B9. Other
C	Change of the direction	From benign to pathogenic, pathogenic to benign (revertant) or normal to more than one pathogenic variant.	C1. Benign to pathogenic mosaicism C2. Pathogenic to benign mosaicism C3. Didymosis C0. If unknown
D	Developmental mechanism	This means the status of the variant cells; heterozygous changes, loss of heterozygosity or epigenetic variants. This attribute includes type 1 and type 2 postzygotic segmental mosaicism, the functional mosaicism of epigenetic mutations, and the lethal autosomal mutations surviving as mosaics.	D1. Type 1 segmental mosaicism D2. Segmental mosaicism with an early second hit including type 2 segmental mosaicism of autosomal dominant traits D3. Type 3 Functional X-chromosome mosaicism with or without male lethality D4. Type 4. Disorders that manifest only as mosaics 4a (Autosomal) / 4b (X-linked) D0. If unknown
E	Etiology	This refers to the size of the genetic/genomic variation or when the change does not affect the size but the functionality of the genome due to epigenetic or positional effects.	E1. Genomic changes (large variations) E2. Genetic changes (small variations) E3. Epigenetic changes E4. Positional effect variants E0. If unknown
F	Fraction of the affected tissue	The percentage of the affected tissue in comparison with normal tissue. See text for definitions of these ranges.	F1. Mild involvement F2. Moderate involvement F3. Severe involvement F4. Very severe (extreme) involvement F0. If unknown.