Each of the 3 NOS isoforms is drawn in monomeric form with the principle domains color-coded, along with the splice variants 2, β, γ, and μ. The term "neuronal" designates only structural affinity to NOS1; for example, splice variant nNOSμ is expressed almost exclusively in skeletal and cardiac muscle. nNOS isoforms have an N-terminal PDZ domain and a PIN (Protein Inhibitor of NOS) association site (aa228-244). PDZ domains target nNOS isoforms, to post synaptic membrane sites. eNOS fatty acylation sites (M = myristoylate, P = palmitoylate) perform an analogous membrane localization function, specifically directing eNOS to caveolae. iNOS and the PDZ-deleted variants of nNOS appear to be soluble and cytosolic. Key amino acid residues in each sequence are numbered; metal ion-coordinating cysteine thiols are indicated by an S. The auto-inhibitory loop of e/nNOS is displaced by binding of Calmodulin (CaM); calculated MW of each isoform/variant is on the left margin.