diff --git a/.gitignore b/.gitignore
index cf54758..471eb00 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 dist
-env
\ No newline at end of file
+env
+*.pyc
\ No newline at end of file
diff --git a/sequence_layers/mlx/__init__.py b/sequence_layers/mlx/__init__.py
new file mode 100644
index 0000000..5587b4b
--- /dev/null
+++ b/sequence_layers/mlx/__init__.py
@@ -0,0 +1,16 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Sequence layers in MLX."""
+
+from sequence_layers.mlx.basic_types import *
\ No newline at end of file
diff --git a/sequence_layers/mlx/basic_types.py b/sequence_layers/mlx/basic_types.py
new file mode 100644
index 0000000..ac518e1
--- /dev/null
+++ b/sequence_layers/mlx/basic_types.py
@@ -0,0 +1,117 @@
+"""Basic sequence types for MLX."""
+
+from typing import Generic, TypeVar
+import mlx.core as mx
+import numpy as np
+
+# A rank 2+ tensor of any type.
+# Note: MLX does not support jaxtyping-style shape annotations out of the box,
+# so we simply bind to mx.array.
+ValuesT = TypeVar('ValuesT', bound=mx.array)
+
+# You can also add the others if you need them:
+MaskT = TypeVar('MaskT', bound=mx.array)
+LengthsT = TypeVar('LengthsT', bound=mx.array)
+ExpandedMaskT = TypeVar('ExpandedMaskT', bound=mx.array)
+# A "self" type alias to allow Sequence and subclasses to return their own
+# Sequence subtype.
+SequenceSelf = TypeVar('SequenceSelf', bound='Sequence')
+Shape = tuple[int, ...]
+DType = np.dtype
+
+
+def sequence_mask(lengths: LengthsT, maxlen: int) -> MaskT:
+  return mx.arange(maxlen)[None, :] < mx.array(lengths)[:, None]
+
+
+class ChannelSpec:
+  """A specification for the channel shape and dtype of a sequence."""
+
+  shape: Shape
+  dtype: DType
+
+
+class Sequence(Generic[ValuesT, MaskT]):
+  """A generic sequence container that preserves masking information."""
+
+  values: ValuesT
+  mask: MaskT
+
+  def __init__(self, values: ValuesT, mask: MaskT):
+    self.values = values
+    self.mask = mask
+
+  @property
+  def shape(self) -> Shape:
+    """Returns the shape of the sequence values."""
+    return self.values.shape
+
+  @property
+  def ndim(self) -> int:
+    """Returns the rank of the sequence values."""
+    return self.values.ndim
+
+  @property
+  def channel_shape(self) -> Shape:
+    """Returns the channel shape (the shape without batch and time)."""
+    return self.values.shape[2:]
+
+  @property
+  def channel_spec(self) -> ChannelSpec:
+    """Returns a "spec" for this sequence (the channel shape and dtype)."""
+    return ChannelSpec(self.channel_shape, self.dtype)
+
+  @property
+  def dtype(self) -> DType:
+    """Returns the dtype of the sequence values."""
+    return self.values.dtype
+
+  def expanded_mask(self) -> ExpandedMaskT:
+    """Returns the Sequence mask with dimensions expanded to match values."""
+    return self.mask.reshape(self.mask.shape + (1,) * (self.values.ndim - 2))
+
+  def mask_invalid(self, mask_value: complex | None = None) -> 'Sequence':
+    """Returns a sequence with invalid timesteps replaced with mask_value."""
+    raise NotImplementedError('Replaced below.')
+
+  def unmask(self) -> 'Sequence':
+    """Returns an unmasked version of this sequence with unchanged values."""
+    # We are already an unmasked sequence.
+    return self
+
+
+class MaskedSequence(Sequence[ValuesT, MaskT]):
+  """Sequence whose invalid timesteps are masked to zero."""
+
+  def mask_invalid(self, mask_value: complex | None = None) -> 'Sequence':
+    """Returns a sequence with invalid timesteps replaced with mask_value."""
+    if mask_value is None:
+      return self
+    else:
+      return mask_invalid(self, mask_value)
+
+  def unmask(self) -> Sequence:
+    """Returns an unmasked version of this sequence with unchanged values."""
+    return Sequence(self.values, self.mask)
+
+
+def mask_invalid(
+    sequence: Sequence,
+    mask_value: complex | None = None,
+) -> 'Sequence':
+  """Returns a sequence whose invalid timesteps are replaced with mask_value."""
+  expanded_mask = sequence.expanded_mask()
+  if mask_value is None:
+    masked_values = mx.zeros_like(sequence.values)
+    result_type = MaskedSequence
+  else:
+    masked_values = mx.full(
+        sequence.values.shape, mask_value, sequence.values.dtype
+    )
+    result_type = Sequence
+  masked_values = mx.where(expanded_mask, sequence.values, masked_values)
+  return result_type(masked_values, sequence.mask)
+
+
+# Defined outside of Sequence so that mask_invalid can return a MaskedSequence.
+Sequence.mask_invalid = mask_invalid
diff --git a/sequence_layers/mlx/types_test.py b/sequence_layers/mlx/types_test.py
new file mode 100644
index 0000000..a9279aa
--- /dev/null
+++ b/sequence_layers/mlx/types_test.py
@@ -0,0 +1,30 @@
+import mlx.core as mx
+import numpy as np
+import sequence_layers.mlx.basic_types as types
+from absl.testing import parameterized
+from absl.testing import absltest
+
+class TypesTest(parameterized.TestCase):
+
+  @parameterized.named_parameters(
+      ('mask_value=None', 0.0),
+      ('mask_value=0.0', 0.0),
+      ('mask_value=-1.0', -1.0),
+  )
+  def test_mask_invalid(self, mask_value):
+    values = mx.array([
+        [1.0, 2.0, 3.0, 4.0],
+        [10.0, 20.0, 30.0, 40.0],
+    ])
+    mask = mx.array([[True, True, False, False], [False, False, False, True]])
+
+    output = types.Sequence(values, mask).mask_invalid(mask_value)
+    expected_values = mx.array([
+            [1.0, 2.0, mask_value, mask_value],
+            [mask_value, mask_value, mask_value, 40.0],
+        ])
+    self.assertTrue(np.allclose(output.values, expected_values))
+    self.assertTrue(np.array_equal(output.mask, mask))
+
+if __name__ == '__main__':
+  absltest.main()