Clarify binders vs entry context on the chain page

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/components/ui.tsx

@@ -71,7 +71,14 @@ export function FidelityBadge({ value }: { value: string | null | undefined }) {
 export function LigandBadge({ status }: { status: string | null | undefined }) {
   if (!status) return <span className="muted">—</span>;
   return (
-    <span className="fid">
+    <span
+      className="fid"
+      title={
+        status === 'holo'
+          ? 'Holo: at least one small-molecule binder on this chain (DNA/RNA not counted)'
+          : 'Apo: no small-molecule binder on this chain (DNA/RNA not counted)'
+      }
+    >
       <Swatch color={status === 'holo' ? 'var(--holo)' : 'var(--apo)'} shape="dot" />
       {status === 'holo' ? 'Holo' : 'Apo'}
     </span>

frontend/src/pages/ChainPage.tsx

@@ -352,13 +352,26 @@ export function ChainPage() {
             pfamIds={c.pfam_ids}
           />
         </Section>
-        <Section title="Ligands & context">
+        <Section
+          title="Binders & context"
+          kicker="Binders are small molecules on this chain; nucleic acid and assembly describe the whole PDB entry."
+        >
           <dl className="facts">
-            <Fact label="Ligand state">
-              <LigandBadge status={c.binding_status} />
+            <Fact label="Small-molecule binders (this chain)">
+              {c.binders.length ? `${c.binders.length}` : 'None'}
+            </Fact>
+            <Fact label="Apo / holo" hint="Holo = at least one small-molecule binder on this chain; DNA/RNA are not counted">
+              <span className="fact-note">
+                <LigandBadge status={c.binding_status} />
+                <small className="muted">by small molecules only</small>
+              </span>
+            </Fact>
+            <Fact label="Nucleic acid in entry" hint="Non-protein polymer present anywhere in the PDB entry, not necessarily bound to this chain">
+              {c.nucleic_acid_binding && c.nucleic_acid_binding !== 'None' ? c.nucleic_acid_binding : 'None'}
+            </Fact>
+            <Fact label="Assembly (entry)" hint="Protein chain composition of the PDB entry">
+              {capitalize(c.chain_composition)}
             </Fact>
-            <Fact label="Nucleic acid">{c.nucleic_acid_binding && c.nucleic_acid_binding !== 'None' ? c.nucleic_acid_binding : 'None'}</Fact>
-            <Fact label="Assembly">{capitalize(c.chain_composition)}</Fact>
           </dl>
           {c.binders.length > 0 ? (
             <div className="binders">

frontend/src/styles/pages.css

@@ -1984,3 +1984,11 @@
   stroke-width: 3px;
   pointer-events: none;
 }
+
+.fact-note {
+  display: inline-flex;
+  align-items: baseline;
+  gap: 8px;
+  flex-wrap: wrap;
+  justify-content: flex-end;
+}