During the last two decades, considerable information has become available on the regulation of gene expression in eukaryotes, although in the 1960's only bacteria were considered suitable for such studies. Due to inherent difficulties in working with multicellular eukaryotes, initial studies in these systems were restricted to the following : (i) formulation of theoretical models for the regulation of gene expression and (ii) conducting experiments to identify whether histones or non-histone chromosomal proteins are relatively more important for the regulation of the activity of specific genes in time and space (i.e. at specific developmental stage and in specific tissue). More recently (particularly after 1980), regulation of specific genes has been examined in greater detail giving valuable information. Specific DNA sequences and proteins (e.g. transcription factors) involved in regulation of specific genes have been identified. These specific genes express either in specialized cells or respond to specific stimuli and are variously described as luxury genes or smart genes as opposed to house keeping genes, which are constitutive in their expression.