Table 1.

The 13 ways of reducing a 4-letter state space ($\mathcal{S}$) to a 3- or 2-letter state space ${\mathcal{S}}^{\prime }$.

Nucleotide-grouping Subsets	Recoding notation	Resulting ${\mathcal{S}}^{\prime }$	Major grouping category
{{A, G}, C, T}	R	{R, C, T}	{2 : 1 : 1}
{A, G, {C, T}}	Y	{A, G, Y}
{A, {C, G}, T}	S	{A, S, T}
{C, G, {A, T}}	W	{C, G, W}
{A, C, {G, T}}	M	{M, G, T}
{C, {A, G, T}}	K	{A, C, K}
{A, {C, G, T}}	B	{A, B}	{3 : 1}
{C, {A, G, T}}	D	{C, D}
{G, {A, C, T}}	H	{G, H}
{T, {A, C, G}}	V	{T, V}
{{A, G}, {C, T}}	RY	{R, Y}	{2 : 2}
{{A, T}, {C, G}}	SW	{S, W}
{{A, C}, {G, T}}	KM	{K, M}