A binomial cell-marking process in the mitotic development of an organism is defined. Basic identities are established for the expectations, variances and covariances of the percentages of type-1 marked cells in general aggregates of cells. The identities are stated in terms of general probabilities that two cells, randomly picked from the aggregates, are in the same marked clone. The Nesbitt model for the X-chromosome inactivation marker and tissue differentiation is generalized in terms of these probabilities. The simultaneous use of two markers is analysed in similar terms. The application of the approach to a variety of aggregates of the haemopoietic system is briefly reviewed and discussed.