diff --git a/src/yadism/esf/exs.py b/src/yadism/esf/exs.py
index faf1f2a8..82c47d62 100644
--- a/src/yadism/esf/exs.py
+++ b/src/yadism/esf/exs.py
@@ -66,6 +66,8 @@ def xs_coeffs(kind, y, x=None, Q2=None, params=None):
         ym = 0
         yp = 1.0
         yL = y**2 / (2 * (y**2 / 2 + (1 - y) - (mn * x * y) ** 2 / Q2))
+    elif kind == "F1":
+        return np.array([1 / (2 * x), -1 / (2 * x), 0])
     elif kind == "XSFPFCC":
         INV_GEV_TO_PB = GEV_CM2_CONV / 100.0  # pb
         norm = (INV_GEV_TO_PB * params["GF"] ** 2) / (2.0 * np.pi)
diff --git a/src/yadism/observable_name.py b/src/yadism/observable_name.py
index dcdc6678..35c57f28 100644
--- a/src/yadism/observable_name.py
+++ b/src/yadism/observable_name.py
@@ -9,6 +9,7 @@
     "XSNUTEVCC",
     "XSNUTEVNU",
     "FW",
+    "F1",
     "XSFPFCC",
 ]
 kinds = sfs + xs + [fake_kind]
diff --git a/tests/yadism/esf/test_exs.py b/tests/yadism/esf/test_exs.py
index b7974595..a9b5d99d 100644
--- a/tests/yadism/esf/test_exs.py
+++ b/tests/yadism/esf/test_exs.py
@@ -50,6 +50,17 @@ def test_xs_coeffs():
     assert fw[0] == 1.0
     assert fw[2] == 0.0
 
+    f1 = xs_coeffs(
+        "F1",
+        x=0.5,
+        Q2=1.0,
+        y=0.0,
+        params=dict(projectilePID=1, M2target=0.0, M2W=1.0),
+    )
+    assert f1[0] == 1.0
+    assert f1[1] == -1.0
+    assert f1[2] == 0.0
+
     xsfpfcc = xs_coeffs(
         "XSFPFCC",
         x=0.5,