diff --git a/release-notes/10.0/preview/rc1/libraries.md b/release-notes/10.0/preview/rc1/libraries.md
index 0753b14a22..b3507d3b20 100644
--- a/release-notes/10.0/preview/rc1/libraries.md
+++ b/release-notes/10.0/preview/rc1/libraries.md
@@ -3,6 +3,47 @@
 The following .NET 10 RC 1 release notes describe new features and changes in
 this release.
 
+## Cryptography: ML-DSA External Mu
+
+The ML-DSA class now allows signatures to be created and verified from an "external" mu (μ) value.
+
+```csharp
+namespace System.Security.Cryptography;
+
+public partial class MLDsa
+{
+    public byte[] SignMu(byte[] externalMu);
+    public void SignMu(ReadOnlySpan<byte> externalMu, Span<byte> destination);
+    public byte[] SignMu(ReadOnlySpan<byte> externalMu);    
+ 
+    public bool VerifyMu(byte[] externalMu, byte[] signature);
+    public bool VerifyMu(ReadOnlySpan<byte> externalMu, ReadOnlySpan<byte> signature);    
+ 
+    protected abstract void SignMuCore(ReadOnlySpan<byte> externalMu, Span<byte> destination);
+    protected abstract bool VerifyMuCore(ReadOnlySpan<byte> externalMu, ReadOnlySpan<byte> signature);
+}
+```
+
+External mu signatures require platform support.
+If, as more providers come online, we find that the provider support is fragmented,
+we may find it useful to add a capability API (e.g. `public bool SupportsMu { get; }`).
+But, for now, we are presenting it as a universally available feature.
+
+## Cryptography: Post Quantum Cryptography "API Complete"
+
+The `MLDsa`, `MLKem`, `SlhDsa`, and `CompositeMLDsa` types (and their associated -Algorithm types) have all been through the API Review process,
+compared against each other for local consistency, and had all of the members declared for the RC1 release --
+both for the .NET 10 built-in versions and for the downlevel compatibility versions in Microsoft.Bcl.Cryptography.
+
+The `MLKem` type is no longer marked as `[Experimental]`, but a handful of methods on it still are,
+because they are based on standards that haven't hit final version/publication.
+We expect to similarly move `[Experimental]` from the `MLDsa` type down to a handful of its members for the stable release of .NET 10.
+The `SlhDsa` and `CompositeMLDsa` classes, however, will retain their `[Experimental]` decoration.
+
+While we have no active plans to introduce breaking changes beyond the stable release of .NET 10,
+the `[Experimental]` attribute is indicating areas where specifications are not complete
+or we are still waiting on features in our underlying platform libraries.
+
 ## What's new features
 
-TBD
+Something about the feature