Table 1.

Proportions of Essential Genes in Different Gene Categories in the Two Phenotypical Data Sets for Mouse.

	Proportion of Essential Genes (%)
Categories	Current Data Seta	Data from Makino et al. (2009)
All genes	43.3	42.07
Duplicates	43.9 (41.6b)	41.92
Singletons	41.1	42.61
All developmental genes	62.53	59.51
Developmental duplicates	64.75	60.9
Developmental singletons	53.1	53.36
Old duplications (Ksc ≥ 2)	47.31	44.94

^a MGI 4.4 (October 2010).

^b If only genes with valid phyletic ages are used.

^c If a gene has multiple duplicates, all pairwise K_s (the number of synonymous substitutions per synonymous site) between this gene and its duplicates will be calculated, and the lowest K_s value is used. Synonymous substitutions in most genes with K_s ≥ 2 will have reached saturation, and hence, the corresponding genes will tend to be older than genes with K_s < 2.