diff --git a/align.c b/align.c
index b56bde2..5d6200d 100644
--- a/align.c
+++ b/align.c
@@ -6,18 +6,19 @@
 #include "mmpriv.h"
 #include "ksw2.h"
 
-static void ksw_gen_simple_mat(int m, int8_t *mat, int8_t a, int8_t b)
+static void ksw_gen_simple_mat(int m, int8_t *mat, int8_t a, int8_t b, int8_t sc_ambi)
 {
 	int i, j;
 	a = a < 0? -a : a;
 	b = b > 0? -b : b;
+	sc_ambi = sc_ambi > 0? -sc_ambi : sc_ambi;
 	for (i = 0; i < m - 1; ++i) {
 		for (j = 0; j < m - 1; ++j)
 			mat[i * m + j] = i == j? a : b;
-		mat[i * m + m - 1] = 0;
+		mat[i * m + m - 1] = sc_ambi;
 	}
 	for (j = 0; j < m; ++j)
-		mat[(m - 1) * m + j] = 0;
+		mat[(m - 1) * m + j] = sc_ambi;
 }
 
 static inline void mm_seq_rev(uint32_t len, uint8_t *seq)
@@ -434,7 +435,7 @@ static void mm_align1(void *km, const mm_mapopt_t *opt, const mm_idx_t *mi, int
 
 	r2->cnt = 0;
 	if (r->cnt == 0) return;
-	ksw_gen_simple_mat(5, mat, opt->a, opt->b);
+	ksw_gen_simple_mat(5, mat, opt->a, opt->b, opt->sc_ambi);
 	bw = (int)(opt->bw * 1.5 + 1.);
 
 	if (is_sr && !(mi->flag & MM_I_HPC)) {
@@ -652,7 +653,7 @@ static int mm_align1_inv(void *km, const mm_mapopt_t *opt, const mm_idx_t *mi, i
 	if (ql < opt->min_chain_score || ql > opt->max_gap) return 0;
 	if (tl < opt->min_chain_score || tl > opt->max_gap) return 0;
 
-	ksw_gen_simple_mat(5, mat, opt->a, opt->b);
+	ksw_gen_simple_mat(5, mat, opt->a, opt->b, opt->sc_ambi);
 	tseq = (uint8_t*)kmalloc(km, tl);
 	mm_idx_getseq(mi, r1->rid, r1->re, r2->rs, tseq);
 	qseq = r1->rev? &qseq0[0][r2->qe] : &qseq0[1][qlen - r2->qs];
diff --git a/ksw2_extd2_sse.c b/ksw2_extd2_sse.c
index 628f37f..a993a2d 100644
--- a/ksw2_extd2_sse.c
+++ b/ksw2_extd2_sse.c
@@ -76,7 +76,7 @@ void ksw_extd2_sse(void *km, int qlen, const uint8_t *query, int tlen, const uin
 	qe2_    = _mm_set1_epi8(q2 + e2);
 	sc_mch_ = _mm_set1_epi8(mat[0]);
 	sc_mis_ = _mm_set1_epi8(mat[1]);
-	sc_N_   = _mm_set1_epi8(-e2);
+	sc_N_   = mat[m*m-1] == 0? _mm_set1_epi8(-e2) : _mm_set1_epi8(mat[m*m-1]);
 	m1_     = _mm_set1_epi8(m - 1); // wildcard
 
 	if (w < 0) w = tlen > qlen? tlen : qlen;
diff --git a/ksw2_exts2_sse.c b/ksw2_exts2_sse.c
index d8178e1..f090af6 100644
--- a/ksw2_exts2_sse.c
+++ b/ksw2_exts2_sse.c
@@ -71,7 +71,7 @@ void ksw_exts2_sse(void *km, int qlen, const uint8_t *query, int tlen, const uin
 	qe_     = _mm_set1_epi8(q + e);
 	sc_mch_ = _mm_set1_epi8(mat[0]);
 	sc_mis_ = _mm_set1_epi8(mat[1]);
-	sc_N_   = _mm_set1_epi8(-e);
+	sc_N_   = mat[m*m-1] == 0? _mm_set1_epi8(-e) : _mm_set1_epi8(mat[m*m-1]);
 	m1_     = _mm_set1_epi8(m - 1); // wildcard
 
 	tlen_ = (tlen + 15) / 16;
diff --git a/ksw2_extz2_sse.c b/ksw2_extz2_sse.c
index c0671a1..88bf636 100644
--- a/ksw2_extz2_sse.c
+++ b/ksw2_extz2_sse.c
@@ -65,7 +65,7 @@ void ksw_extz2_sse(void *km, int qlen, const uint8_t *query, int tlen, const uin
 	flag16_ = _mm_set1_epi8(0x10);
 	sc_mch_ = _mm_set1_epi8(mat[0]);
 	sc_mis_ = _mm_set1_epi8(mat[1]);
-	sc_N_   = _mm_set1_epi8(-e);
+	sc_N_   = mat[m*m-1] == 0? _mm_set1_epi8(-e) : _mm_set1_epi8(mat[m*m-1]);
 	m1_     = _mm_set1_epi8(m - 1); // wildcard
 	max_sc_ = _mm_set1_epi8(mat[0] + (q + e) * 2);
 
diff --git a/main.c b/main.c
index 4dc7205..719ecd4 100644
--- a/main.c
+++ b/main.c
@@ -10,7 +10,7 @@
 #include "getopt.h"
 #endif
 
-#define MM_VERSION "2.10-r768-dirty"
+#define MM_VERSION "2.10-r770-dirty"
 
 #ifdef __linux__
 #include <sys/resource.h>
@@ -58,6 +58,7 @@ static struct option long_options[] = {
 	{ "min-occ-floor",  required_argument, 0, 0 },   // 28
 	{ "MD",             no_argument,       0, 0 },   // 29
 	{ "lj-min-ratio",   required_argument, 0, 0 },   // 30
+	{ "score-N",        required_argument, 0, 0 },   // 31
 	{ "help",           no_argument,       0, 'h' },
 	{ "max-intron-len", required_argument, 0, 'G' },
 	{ "version",        no_argument,       0, 'V' },
@@ -175,6 +176,7 @@ int main(int argc, char *argv[])
 		else if (c == 0 && long_idx ==28) opt.min_mid_occ = atoi(optarg); // --min-occ-floor
 		else if (c == 0 && long_idx ==29) opt.flag |= MM_F_OUT_MD; // --MD
 		else if (c == 0 && long_idx ==30) opt.min_join_flank_ratio = atof(optarg); // --lj-min-ratio
+		else if (c == 0 && long_idx ==31) opt.sc_ambi = atoi(optarg); // --score-N
 		else if (c == 0 && long_idx == 14) { // --frag
 			yes_or_no(&opt, MM_F_FRAG_MODE, long_idx, optarg, 1);
 		} else if (c == 0 && long_idx == 15) { // --secondary
diff --git a/minimap.h b/minimap.h
index d87c891..65f49c3 100644
--- a/minimap.h
+++ b/minimap.h
@@ -118,6 +118,7 @@ typedef struct {
 	float min_join_flank_ratio;
 
 	int a, b, q, e, q2, e2; // matching score, mismatch, gap-open and gap-ext penalties
+	int sc_ambi; // score when one or both bases are "N"
 	int noncan;      // cost of non-canonical splicing sites
 	int zdrop, zdrop_inv;   // break alignment if alignment score drops too fast along the diagonal
 	int end_bonus;
diff --git a/minimap2.1 b/minimap2.1
index 0208ea9..5856d25 100644
--- a/minimap2.1
+++ b/minimap2.1
@@ -1,4 +1,4 @@
-.TH minimap2 1 "27 March 2018" "minimap2-2.10 (r761)" "Bioinformatics tools"
+.TH minimap2 1 "30 April 2018" "minimap2-2.10-dirty (r770)" "Bioinformatics tools"
 .SH NAME
 .PP
 minimap2 - mapping and alignment between collections of DNA sequences
@@ -322,6 +322,9 @@ no attempt to match GT-AG [n]
 .BI --end-bonus \ INT
 Score bonus when alignment extends to the end of the query sequence [0].
 .TP
+.BI --score-N \ INT
+Score of a mismatch involving ambiguous bases [1].
+.TP
 .BR --splice-flank = yes | no
 Assume the next base to a
 .B GT
diff --git a/options.c b/options.c
index 5c19978..aa48ae4 100644
--- a/options.c
+++ b/options.c
@@ -34,6 +34,7 @@ void mm_mapopt_init(mm_mapopt_t *opt)
 	opt->min_join_flank_ratio = 0.5f;
 
 	opt->a = 2, opt->b = 4, opt->q = 4, opt->e = 2, opt->q2 = 24, opt->e2 = 1;
+	opt->sc_ambi = 1;
 	opt->zdrop = 400, opt->zdrop_inv = 200;
 	opt->end_bonus = -1;
 	opt->min_dp_max = opt->min_chain_score * opt->a;
diff --git a/python/cmappy.pxd b/python/cmappy.pxd
index 231f68f..39be8a4 100644
--- a/python/cmappy.pxd
+++ b/python/cmappy.pxd
@@ -26,6 +26,7 @@ cdef extern from "minimap.h":
 		int min_join_flank_sc
 		float min_join_flank_ratio;
 		int a, b, q, e, q2, e2
+		int sc_ambi
 		int noncan
 		int zdrop, zdrop_inv
 		int end_bonus