Genetic variation and plasma creatine kinase activity

The distribution of plasma creatine kinase (CK; E.C.2.7.3.2) is known to be skewed, and this has made it difficult to analyse the sources of variation. We have studied plasma CK in 206 pairs of twins and have analysed the results after separating them into what appear to be two Gaussian frequency distributions. The results in the main distribution (CK less than 300 iu/1) are apparently affected by genetic factors common to both men and women, and by environmental factors which are of much greater effect in men. The tendency for some men to have very high CK values may also have a genetic basis.