Table 1. Proportion of polypeptides encoded by single-copy genes (singletons).

Protein structure (n = subunit no.)	Total no. of polypeptides studied	No. of singletons	Proportion of singletons, Q	Total no. of gene families	Proportion of singleton families, P
Yeast
Monomers*	754	474	0.629	669	0.709
Monomers†	341	192	0.563	306	0.627
Protein complex subunits‡	1,136	697	0.614	902	0.773
Dimer subunits (n = 2)‡	171	96	0.561	135	0.711
Hetero	75	43	0.573	62	0.694
Homo	96	53	0.552	73	0.726
Midsize complex subunits (3 ≤ n ≤ 10)‡	278	177	0.637	231	0.766
Hetero	216	150	0.694	183	0.820
Homo	62	27	0.435	48	0.563
Large complex subunits (n > 10)	196	160	0.816	183	0.874
Human
Monomers (n = 1)	198	33	0.167	161	0.205
Dimer subunits (n = 2)‡	1,492	141	0.095	555	0.254
Hetero	916	52	0.057	208	0.250
Hetero§	358	52	0.145	205	0.254
Homo	372	69	0.185	258	0.267
Midsize complex subunits (3 ≤ n ≤ 10)‡	958	156	0.163	521	0.299
Hetero*	538	97	0.180	313	0.310
Homo	233	40	0.171	153	0.261
Large complex subunits (n > 10)‡	377	128	0.340	295	0.434
All protein complex subunits‡	2,963	453	0.153	1,399	0.324

For yeast, data for monomers and protein complex subunits are from MIPS; data from dimer subunits, midsize complex subunits, and large complex subunits are from Swiss-Prot.

^*Monomers, proteins of no recorded interaction.

^†Monomers after excluding unclassified (unnamed) genes, which were genes that had the same names as their ORF names in the Saccharomyces Genome Database.

^‡We excluded all ambiguous cases where a protein complex can be, for example, both a dimer and a trimer. Therefore, the total number of protein complex subunits is larger than the sum of the three groups of protein complex subunits of different size. The same rule was applied to the classification of heteromers and homomers.

^§After excluding the three supergene families (558 genes) related to the immune system.