diff --git a/src/gp/genome.rs b/src/gp/genome.rs
index 802d668..14ed65c 100644
--- a/src/gp/genome.rs
+++ b/src/gp/genome.rs
@@ -29,7 +29,7 @@ pub fn make_random_plushy(
 }
 
 /// A map of genes to their number of blocks they open.
-static OPEN_MAP: LazyLock<HashMap<Gene, u8>> = LazyLock::new(|| {
+pub static OPEN_MAP: LazyLock<HashMap<Gene, u8>> = LazyLock::new(|| {
     let mut temp_map = HashMap::default();
     temp_map.insert(StateFunc(exec_dup), 1u8);
     temp_map.insert(StateFunc(exec_dup_times), 1u8);
diff --git a/src/gp/utils.rs b/src/gp/utils.rs
index e497d32..c7fed49 100644
--- a/src/gp/utils.rs
+++ b/src/gp/utils.rs
@@ -1,4 +1,5 @@
 use crate::gp::args::ClosingType;
+use crate::gp::genome::OPEN_MAP;
 use crate::push::state::Gene;
 use rand::Rng;
 use rand::seq::IndexedRandom;
@@ -12,9 +13,57 @@ pub fn random_instruction(
     match closing_type {
         ClosingType::Specified => return instructions.choose(rng).unwrap().clone(),
         ClosingType::Balanced => {
-            let 
+            let source: Vec<Gene> = instructions
+                .iter()
+                .filter(|instr| !matches!(instr, Gene::Close))
+                .cloned()
+                .collect();
+            let total_opens: usize = source
+                .iter()
+                .filter_map(|instr| {
+                    if let Gene::StateFunc(_) = instr {
+                        OPEN_MAP.get(instr).copied().map(|val| val as usize)
+                    } else {
+                        None
+                    }
+                })
+                .sum();
+            let p = if source.is_empty() {
+                0.0
+            } else {
+                total_opens as f64 / source.len() as f64
+            };
+            // Return Close or a random instruction based on probability
+            if rng.random::<f64>() < p {
+                Gene::Close
+            } else {
+                source.choose(rng).unwrap().clone()
+            }
+        }
+        ClosingType::None => {
+            // Find multi-block instructions (those with opens > 1)
+            let multi_block_instructions: Vec<Gene> = instructions
+                .iter()
+                .filter(|instr| {
+                    if let Gene::StateFunc(_) = instr {
+                        OPEN_MAP.get(instr).map_or(false, |&opens| opens > 1)
+                    } else {
+                        false
+                    }
+                })
+                .cloned()
+                .collect();
+
+            // Remove Close and multi-block instructions
+            let source: Vec<Gene> = instructions
+                .into_iter() // Take ownership of instructions
+                .filter(|instr| {
+                    !matches!(instr, Gene::Close) && !multi_block_instructions.contains(instr)
+                })
+                .collect();
+
+            source.choose(rng).unwrap().clone()
         }
-        _ => todo!(),
     }
 }
 
@@ -33,3 +82,34 @@ pub fn gaussian_noise_factor(rng: &mut impl Rng) -> Decimal {
 
     (dec!(-2.0) * u0.ln()).sqrt().unwrap() * (dec!(2.0) * rust_decimal::Decimal::PI * u1).cos()
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::instructions::code::*;
+    use crate::instructions::common::boolean_rotate;
+    use crate::instructions::vector::*;
+    use crate::push::utils::most_genes;
+    use rand::SeedableRng;
+    use rand::rngs::StdRng;
+
+    #[test]
+    fn random_instruction_test() {
+        let mut rng = StdRng::seed_from_u64(42);
+        let genes = most_genes();
+
+        let rand_instruction = random_instruction(genes.clone(), ClosingType::Specified, &mut rng);
+        assert_eq!(
+            Gene::StateFunc(vector_float_from_last_prim),
+            rand_instruction
+        );
+
+        let mut rng = StdRng::seed_from_u64(32038);
+        let rand_instruction = random_instruction(genes.clone(), ClosingType::Balanced, &mut rng);
+        assert_eq!(Gene::StateFunc(boolean_rotate), rand_instruction);
+
+        let mut rng = StdRng::seed_from_u64(3203890821);
+        let rand_instruction = random_instruction(genes, ClosingType::None, &mut rng);
+        assert_eq!(Gene::StateFunc(code_insert), rand_instruction);
+    }
+}