A proposed classification of genetically determined mosaicism in man.

Terms are often used in medical literature without a true awareness of their original meaning and previous usage. Although the recent advances in human cytogenetics have focused attention on the phenomenon of mosaicism, in fact the term 'mosaic' was used in biology and human genetics long before human cytogenetics was a recognized discipline. The term 'mosaic' was borrowed, originally as a purely descriptive term, from the realm of fine arts. The term in its original context leaves no room for confusion or ambiguity. It is defined as 'the process of producing pictures or patterns by cementing together small pieces of stone or glass, etc. of various colours; pictures or patterns thus produced; the constructive or decorative materials of these' (Oxford Shorter Dictionary). This clarity of its meaning seems to make it a particularly appropriate descriptive term for certain naturally occurring patterned organisms (pathological or normal) where living tissues show the clear-cut difference of a mosaic. When used outside the realm of the inanimate it should always be thought of as 'mosaic-like', and any definition or classification should encompass all such mosaic-like situations regardless of their origin or level of observation. Such situations are sufficiently rare to attract widespread interest. They have been extensively investigated in plants (see Cramer, I954 and Neilson-Jones, 1937, for reviews). More recently general discussions of their occurrence in the animal kingdom (Hannah-Alava, I960) and in mice (Russell, I964) have been published. Recent discussions of the subject (Lancet, I965; Bain and Scott, I965; Engel, I965) have tended to ignore the diversity of origin of mosaics, and the purpose of the present communication is to propose a general classification of mosaicism in man.